Converting friction shifters to SIS or STI shifters Edit: I'm now worried the main issue might be if it's at all possible to mount cable stops to the top mount position the cables exit the DT into the shifters.
As I mentioned, I'm not there to take more photos at the moment, but you could see a bit better if you open the photo attached to max size.
I do actually prefer STI, realized this after posting. need to understand if that's ruled out, and if so - if there are any SIS shifters that would fit as replacement.
I hope there's a solution that doesn't require serious engineering.

Edit 2:
Attaching pics I found for someone with roughly the same bike who performed the same conversion I'm trying to make.
It requires some machining, but if I can do that, it might be the better option in my opinion over SIS. *I still don't even know if I can get SIS shifters to fit the mount I have.
Also attaching better pictures of my top mounted shifters - fitted on a boss that isn't identical, although the machined part I require is probably the same.
For the part I need to prepare, I'm still waiting to hear from the owner of the bike in the photo, hoping to get a few more angles and possibly dimensions so I can replicate it. I'm curious about the reverse angle, probably showing how the exposed cables go into the new piece.



The last two pics show how the cables are currently routed under the BB

Surprisingly this seems more crude than the guide system on an earlier 80-81 model, with a practically identical frame:

This is continuing another thread I started, asking advice on whether I should buy a used groupset.
The most thorough advice I got there, suggested replacing my 6s freewheel to a 7s one, and couple that with 8s STI shifters - my purpose was actually converting to STI.
I'm now trying to figure out if I might be better overall converting to 7s SIS.
Arguments for 8s STI:

I was told 8s STI shifters should work fine with a 7s freewheel, and that even the people at Shimano said so.
Brifters are easier to use.

Cons:

some people say 8s shifters with a 7s freewheel isn't a perfect match
I'll have to use cable stops, I'm still not sure how to go about this on my frame - the tutorials I saw had the break cables routed outside the frame, not inside it, and they were also positioned at the side of the downtube

Arguments for 7s SIS:
Its all pretty much said above -

Should be no argument about compatibility with a 7s freewheel
I don't have to deal with the cable stop issue, easier installation overall.

Cons:

Less comfortable shifting
My old friction shifters were so finicky that I'm prejudiced against all downtube shifters

Anyone with experience and opinions to contribute?
It would be great to understand if there's anything special I need to take into account considering the position of my current shifters and the routing through the tube.
Thanks
Editing to answer a question I got:
The frame is aluminum, with 126mm spacing. Which means it's not a candidate for cold setting, and therefore can't take anything more than a 7s freewheel - as far as I understand. And as Nathan pointed out in the previous thread I linked to, the dropouts are enormous and might not be great for flexing. I'm not there to measure the actual spacing, but the literature has it as 126
Since you're going to a lot of work, moving to 8 speed (or 9 or 10) sets you up for the most flexibility.
The main reason is that you need a new rear hub with a cassette-freehub.  There's no good way to convert a freewheel based wheel.  Since you're buying a replacement hub/wheel, its no more work to pick an 8~10 speed one.
The OLD of your bike needs to match the hub, so measure that first.  If its 126mm, you'll have to decide if you want to cold-set your frame.  Is your frame steel? It could be aluminium from the photo, which doesn't cold set.

You will need a full set of parts, plus substitutes for the fittings missing on your frame.
Starting at the top, the brifters should just slide onto your drops and bolt up.  You will need to either carefully unwrap the existing bartape to reuse, or buy some new.
Next are the downtube stops. Your frame has nice but unusual shift levers - the more common ones are two lugs horizontally out the sides.  For them its possible to buy bosses that create stop points.

There are other options - you might choose a &quot;band-on&quot; cable stop like this:

or if you're happy with welding/brazing/2-part epoxy then stick-on ones might suit.

Ideally you'll be able to use the existing cable track under the BB, the photo isn't clear how the internal cable exits the downtube.

If you want to somehow tie the new brifters into the existing access holes on top of the downtube, well that's going to need some engineering and likely a frame builder.
One possibility might be to use Nokon-style cable housing to run from your brifters all the way down to the existing cable entry holes, but that could leave some loops of outer that will look weird.  There could also be stresses on the cable as it passes through at least one 90-degree bend, maybe two.


Additionally, I found that the Klein downtube bosses had some kind of weakness in the past.  Details are from https://www.bikeforums.net/bicycle-mechanics/729717-hillrider-s-klein-stage-comp-fix-shifter-boss-bolt-stripping.html
In summary:  the bosses are weak and there's a refit kit to make them stronger.  However the instructions suggest the existing boss is only bolted to the frame, so it may be possible to unbolt, for a cleaner look.
With the knowledge Top-mount downtube shifter I found this rare Suntour adapter - you might choose to use this as inspiration to make your own, or talk to a machinist to get something made:
   from https://www.bikeforums.net/classic-vintage/1163305-how-use-ergo-sti-shifters-aero-dt-shifter-mount.html some gorgeous photos there
And here's someone who secured normal lugs to his Klein:
 from https://www.bicycles.net.au/forums/viewtopic.php?t=30867

Finally, whatever you do, save the take-off pieces.  Someday you or a future owner might want to restore the bike to original, and these levers are rare.  So please don't throw them out.  You might even choose to sell all the takeoff parts to help fund your changes, but don't bin them.