Well, with a $15k budget you can do both if you are doing the work yourself. My first order of priority would be suspension . Brakes / frame connectors / solid bushings, etc. I would rather have a car that turned a few extra RPMs because of the TH350, but drove great. then a overdrive that turned lower RPMs but drove like Doo Doo! You can do some really nice front subframe / rear suspension combos (Speedtech subframe / torque arm this month is on a killer special) for about $6500 or so. You can spend up to $9500 for some systems. I am assuming normal freight charges in those figures. Add $3000 for killer brakes. So even at the top of the list, you still have plenty for a transmission for a 500HP or less combo.
All the thing that I have seen Frank and his crew do lately, He will be the first one that I would call when I get to that stage. I feel that they do great work and sell great parts at good prices.
Call Frank! He can help guide you threw this and sells everything that would be good options for you...therefore is unbiased! Enjoy the project/new car!