1st, welcome to the mustang world
check my vid at teh bottom
its a 66, 3spd with 2.8 gears (much worse than 373s)
a wrx runs mid 13's i believe
my car should run mid 14's
i changed the cam (performer- is pretty mild), new intake/carb and some headers from stock 2barrel 289
i could break the 13 barrier easily with some nice aluminum heads (mine are stock)
heads 1 to 1.5k - look into air flow research
cam $100 - your preference
intake $150 - your preference (weiand is most popular - i like edelbrock)
carb $280 - this price is for a road demon... much better at a slightly higher price
exaust ?? - 2.5" pipes are probably about norm (decent mufflers)
of course you want to address safety issues - this would be most important
if youve got friends who know they're stuff, you can inspect and fix most everything
mainly brakes, and lube/check your suspension, i would also check for rust (hopefully you did this prior to buying)
and a rebuild kit for your motor is going to cost a few hundred
but your early mustang will move handily with just these mods
*reason i wanted you to check vid, is im really proud i finally got it to my cpu
im doing around 50 in the flyby in the first scene from teh complete stop
Its like this...$5k won't get you in the game of running with WRX, LS1, Mach 1 class of machines, but it can get you respect as a good classic, which is what it is. You might want to think about just adding a 4bbl intake and carb, electronic ignition, some headers and Flowmasters, front disc brake conversion, new springs and shocks, and a posi 3:25 rearend. Your car will run much better, ride much better, and will be safer. You can do all the above with a service manual and basic tools. You'll have respectable power, but not enough to break other things along the drivetrain...
If you don't have the cash to build a racer, don't try. If you don't have much experience, start small and build up as your experience grows.
I realize that everyone will give you tons of advice, based upon their experience and what they would do with 5 grand in their pocket. The following is always true:
Everything is harder than it sounds...
Friends never show up when needed most...
You never have enough tools...
It always rains when you're working outside...