If you really want to learn about blowers for a 5.0, I suggest you check out
www.corral.net This is a Mustang site, with very knowledgable people on this subject.
There are quite a few blower companies out there these days, such as Vortech, Powerdyne, ProCharger (my favorite) on the centrifugal side, as well as Eaton (roots) and Kenne-Bell (whipple charger). The ProCharger is probably the hottest setup currently with the Mustang crowd, it's a great product and most come with an intercooler (reduces the temperature of the incoming air into the blower, which means more power).
Talk to as many experienced blower people as you can to learn what will work best for your application. Call and talk to the different blower companies and see if they have a product to fit your needs, plus, I'm sure you'll learn quite a bit about blowers in general in the process.
The roots type or whipple charger will provide more power down lower, but are limited somewhat in the upper rpm range, where max HP is developed. That's why most people serious about serious HP usually go with the centrifugal models. I'd say Vortech and ProCharger are the choice for most HP junkies.
As to your question of "small vs. big" blowers and the amount of boost they run, the "big" models will have more capacity than the "small" models, but the boost is really determined by the size of pulley you use. That's why you'll see ProCharger's P600 model being set up for 9, 11, or 14 psi. And boost is boost. The more you pressurize the intake charge, the more fuel you'll be able to burn, and the more HP you'll be able to make.
The higher you go with boost, the more you'll have to modify supportive systems like air intake and fuel, as well as retard timing so as to avoid detonation. Also, the higher the boost, the higher the octane of fuel required. For your application, you're probably going to go with 6-9 lbs of boost, to make it more streetable. Check with the pros, they'll steer you the right way.
Another power adder that is worthy of consideration is a nitrous setup. While not as versatile as a blower, the cost is MUCH lower, and the HP gains are quite impressive. Nitrous and blowers essentially do the same thing, enable you to burn more fuel in a given space, they just go about it a bit differently. The downside to nitrous is that you'll have to refill the bottle periodically, and most systems only operate at WOT (wide open throttle), although there are computerized systems now that can inject a charge in relation to engine RPM before WOT.
I'd do as much research as possible, and find out what power adder will fit your needs and budget the best.
