Single chainring conversion issues I've got a Specialized Crosstrail Elite. I want to get rid of the front derailleur and go for a single front chainring.

Its a 68mm BB shell but needs a 73mm BB with spacers. It currently has a FSA Modular VDrive Crankset, 44/32T. FSA only do a Modular single chainring which is 38T. I'm looking to go larger, minimum of 44T.

I'm looking at changing the crankset to Shimano, including the BB. If I want a bigger chainring, I'll need to go for a road crankset, but because road cranksets run on a 68mm BB I won't get the clearance at the chainstays. The spindle length isn't as long as MTB cranks.

If I go with an MTB crankset, I can't seem to get a chainring over 38T. Which is two small. I'd also need to run a narrow-wide ring to decrease the risk of chain drop.

I'm going around in circles
You are correct that you may run into clearance problems if you just put on a road crank.

You should get a narrow-wide ring no matter what. There's no reason to use anything else for a 1x.

To get the kind of gearing you want (44t and higher on a mountain 1x crank), the main way of doing it from scratch is get a 1x crank arm set and BB of the sort appropriate for your bike with a 110mm direct mount spider available for it. You You can then use any 110mm narrow-wide chainring, which come in the "road" type sizes you want, and some of which are inexpensive to boot.

Currently what's going on is a bunch of companies have 110mm direct mount spiders and/or cranks that can take them, all based on the SRAM 3-bolt standard. Some examples of that are the Origin8 Thruster crank system and Cane Creek eeWings.

Also, as far as your existing cranks, my understanding is the mounting pattern itself is the same between all the FSA modular cranks, and is also shared by Cannondale SI cranks. There are various FSA direct mount rings that fit it in more "road" type sizes. Some of the fancy 3d type ones like the SLKs may have physical incompatibility with your cranks, or not. But, mostly, I believe the distinguishing factor in what Modular type rings in their line up they say will fit it is about guiding people to wind up with a chainline that makes sense for each crank's application.

Anyway, FSA doesn't seem to offer a 110mm direct mount spider for their Modular cranks, which I find kind of odd, but Cannondale does have them and I assume they'd fit without issue, although I haven't actually tried it. If I were going to try going that route, because you'll be in hack territory, what I would do is get the spider first and then take measurements once it's mounted to determine what type of ring offset and/or spacers to use to get the chainline you want, rather than trying to figure that out in the abstract and order everything at the same time. It would be an experiment, but I bet you could get this approach to work without issue.