Why not start with a 340/360 and a 4.125 stroke crank. 430+ cube small block. 1.5 hp/cube get's you to around 645. Might be a bit more than the block could handle without some help.
Because he already had a 318. Iv watched alot of his videos and that was engine just morphed unto what is it today. He ran it as a 318 then kept adding stuff. I like it I think its the perfect definition of a shade tree muscle car build.