Some sort of ignition retard is needed with any forced air application. You need to retard the timing based on boost pressure. But that has nothing to do with boost.
Several ways you can control boost. Pulley size as mentioned before. The other is a Blow off Valve. I'm not sure if the later would really work though for your applications. It's used more for turbos to protect the turbo and engine from boost spikes during shifts (the turbo lags both before and after giving it gas). I'm sure the same principle applies for a supercharger.
the M90 will be a tough one to mount, even on the driver's side. I looked at it and did some measurements, and the dizzy gets in the way, or will be very close.
Another option, though more expensive is a centrifugal supercharger like a vortech or paxton (I think they are the same company now). They are a little bit smaller and may pose less of a problem.
to your question though, the amount of HP will be dependent upon the amount of boost. In theory, if you were to run 7.2 PSI of boost, you can expect about 50-60% gains. The reason is that a 7.2PSI you are at 150% volumetric efficiency where a NA engine is aroudn 80% VE. you have 70% more air and Fuel, which should translate to around 70% increase in HP...in theory. HP and power in general is linked to how much fuel and air you engine can efficient combust.
so...stock with the modifications you talk about, will get you around 80-90 RWHP. Take 50% of that and you should be aroudn 120-130 RWHP I would guess at 7 PSI.
But one thing I'm unsure, are you going to be using the stock 1v log head? Those estimate are for that. Also, consider doing a hotter cam before supercharger, that will help a lot. Or at least do a cam swap when you put the supercharger in.
Another issue with the M90, is you have to do draw thru with the carb. This can both help and hurt you situation. It means you'll have to remote locate the carb before the supercharger. Most cars get around then by putting the supercharger on top of the manifold and after the carb. This can help you locate the carb and supercharger to a more desirable location. The other way I saw to make it fit without interferance (and I hope you don't have AC or PS, is to move it out from the engine some on the drivers side (about 5 inches from the side of the block) and brace it from there to the engine. Means a long pulley run. mount in sideways, and you should have enough clearance, but you have to figure something out with the carb.
Good luck.
Slade