Going from 7 Speed Cassette to 8 Speed Cassette here is the situation – I have a quasi-vintage 7-speed bike that still has the original front and rear derailleur (Deore DX) in very good condition.  I have recently took a wheel building class which, naturally, resulted in a nice new set of wheels that I would like to use on this bike.  However, the freehub on the new rear hub is for a 8/9/10 cassette.  

Now, I realize that I could buy a spacer and place a 7 speed cassette on that bad boy.  My question is this – Could I put an 8 speed cassette on the new hub, avoiding needing a spacer, and then adjust the limit screws on the rear derailleur to accommodate the extra gear on the cassette?  I’m currently using a set of dura ace 8-speed bar end index shifters at the moment, so that wouldn’t be a concern.  I also believe a 7 speed and 8 speed drivetrain uses the same chain, too.

Thanks for any help!
Dura-Ace 6/7/8-speed used different cable pull ratios from other models. To get your shifter to work properly with your DX rear derailleur you may have to try alternate cable routing. As always, Sheldon has the details: https://www.sheldonbrown.com/dura-ace.html