You can't go very large with internal balance in a 460. I think 501/502 are as big as you can go with internal balance. I don't think the 4.3" stroke cranks are available in internal balance. Unless your going for a manual transmission, external balance would get you the most bang for your $$, because you can get nearly 600 CID for the same price as 500 CID.. Reason for internal balance better for M/T is because a 390 flywheel will work with a 351M/400 4spd bellhousing which can be had cheap, but will not clearance external balance flywheels. You would need that illusive 460/4spd bell from an 80's F-350 that came with a T-19 or an aftermarket bell. Either one isn't cheap
has better prices than Summit, and you can actually message and talk to Carl on 460 Ford Forum
if you need to plan out a good build.