So first off, boost is merely a measure of restriction, and will change from motor to motor, so giving you a "max boost" on a HCI 302 without blowing a head gasket is just a guess. It depends on the block, the fasteners, the head gaskets used, the machined surface on the block/heads, the amount of VOLUME of air that the compressor is pushing, etc. If you put your S-Trim on a HCI 302, and for example it creates 10 pounds of boost, that same blower on a 418 cubic inch motor might only make 3 pounds of boost; however, the
volume of air being pushed remains the same.
I recommend going to
www.turbomustangs.com and reading "the turbo bible." You'll learn about compressors.
Also, everyone seems to think that a stock block will supernova at 500 horsepower, but that's also a myth. At the same website above, you can read the "stock block chronicles" of all the people pushign stupid power levels for a long period of time on a crappy production 5.0 block. It has more to do with tune and RPM than it does a particular power level.
I'm not aware of a "bolt on" air-to-air kit for the S-trim, but feel free to do some googling. An aftermarket bypass valve is probably unnecessary in this application. I believe Vortech publishes their compressor maps too, so you can find out what volume your pushing, and what the adiabatic efficiency is of the compressor at that volume, and the intercooler may very likely be a "nice-to-have" but not required.
A set of 42s would be fine, and they're cheap.
Depending on power levels, you could consider an inline pusher pump to help with fuel volume. There are lots of calculators online to help you figure out fuel requirements.
Ditch the FMU. Get an Anderson Power Pipe. Switch to a fox TB setup. (that tiny charge pipe between the blower outlet and SN95 TB is VERY restrictive) Assemble the entire blower off the car first to ensure you have all the bolts, lines, and fittings you need. You can download the parts list with the instructions online and order whatever else you need. Buy some new plugs and gap them tighter. Get an MSD or other ignition box while you're at it. Ensure you entire ignition system is in good working order.
Oh, and have fun!
Paul.