You may find that it is not as cheap to do a 460 when you have a truck set up for an FE now
You'll need a bell/blow shield, potentially a special flywheel to match, all the external brackets, distributor, water pump, oil pan, intake, balancer, etc, and you still have to build the 460 (although my guess is that the crank will be significantly cheaper, rods and pistons should be about the same)
There is no doubt in my mind that a well-prepped 429 or 460 will out power the FE in that application, but if the 390 is competitive now, building a solid 445 short block might be the smartest way out, and the 445 is a real strong combo. Now if you decide to stroke a 460, power starts climbing quickly, all depends what you want to do
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed