More cylinder head and bigger headers would be much better obviously. I am in the same boat though. I already had heads etc but needed to build a new short block. Well since it costs the same to build 512 with new parts might as well go that route and leave room for more cylinder head later.
I didn't use a kit. I bought a 440source crank, 6.7 rods and a shorter 1.12 piston that's generally for the 4.15. But with the longer 6.7 rod it comes out .010 farther in the hole which can be decked. But the pistons are lighter and the rod ratio is better.