I'd agree with the advice to stick with the 351C. I owned a 64 300 hp 390 and had friends with GT390 Mustangs - good torque but not willing revvers. But, had another friend with a wickedly fast 428 CJ 69, but that's a considerable step up from a 390, unless you go the full route with like CJ heads (hard to find decent shape) or Edelbrock heads, you can make a 410 out of a 390 w/a 428 crank, but it will cost a lot to get real power out of it. If you want a little better throttle response out of your Cleveland, you might consider a set of Aussie 2bbl closed chamber heads from
www.powerheads.com. The 390 also is a heavy motor for that car and it's not very stiff chassis. Weiand Stealth intakes are good hi end performers yet still have good street torque, and there's always aftermarket efi to beef up throttle response even more, w/o losing power (for use on a 4bbl intake) and retrofit hydraulic roller lifters from Crane giving you great cam choices.