If you are building a complete engine from scratch then a bare block is all you need!
You will have to buy the pistons after you select your heads.
67 and 68 engines are the same in theory with close chamber heads.
516 in 66/67 and then 915 in 67/68.
All irrelevant with an aftermarket head.
Most folks are saying the 'trickflows are the best 'bang for the buck' at present?
Some like high comp builds but I prefer under 10 to 1 myself.
As anyone knows when you build a fresh Engine.
Nearly everything that is 50+ years old is trashed...
You only save the block and the crankshaft.
The cost of strip-down and cleaning then checking 'old parts' is expensive!
Buy all new parts and save everyone's time and your money...
I recently built a 1967-440 with ported iron heads and sixpack rods, new forged pistons all new valve gear etc.
Then one of the rods lost grip on the pin and it scored the 'bore' real bad and the whole lower-engine was JUNK!
$3000 build up in smoke for a $25 rod...