Wow. You need some drag radials or other traction device! 107 trap is good for a potential high 12 ET. What are your 60' times looking like? The advantage is more pronounced on DRs, but a steaper gear and practice on it, will probably net you a mph or 2 and a tenth or two in the 1/4. Either way I think a set of slicks or high sidewall DRs (275s on a 15 inch wheel) and a ballsy launch could net you over half a second on the strip.
Personally I have 4.10s for two reasons. 1. When I got them 2 years ago people told me I was crazy for going that high and at the time I don't think there were the speedo correction devices other then the gears. 2. I'm going to supercharge, and even on beefy street tires 400+ rwhp is a lot to control. If you are planning on supercharging then 3.73s may well be enough. If you are staying NA then the nature of the engine is to make power high in the RPM, especially after the typically breathing and exhaulst modifications. All the fast NA guys (low 12s) are running atleast 4.30s and usually higher.
The only after market IMRC product I know of is the FRPP delete kit, which is they remove the throttles and shafts, apoxy the holes and charge you something crazy for your trouble. Anyway for just random knowledge the 98s have a bit better IMRCs then the 96-97s. They're a tiny bit bigger and the throttles are made of a plastic as aposed to aluminum for a bit better heat insulation.
If the Mac pullies don't have a damper then its not the end of the world, but it would make me sleep better if they do. I haven't heard of anyone in a Mustang EVER doing it, but I have seen a few Hondas spin bearings (ouch! and ouch$) because of non-damped pulleys.
I'm using Steeda pulleys which are either relabeled ASP pulleys or the other way round, or they both just look and weigh exactly the same.
Either way, they're bullit steel and have a nice damper.