The 1974 350 block with 4 bolt mains is a good start. But you don't say whether you had the block to a machine shop to be checked. I also assume you didn't have the rods re-sized since you mentioned stock rod bolts. Nothing wrong with stock cast pistons if you checked them making sure they are not out of round or cracks between lands. Intake is fine for 200cc heads. 215cc heads recommend a single plane intake. XE268 cam is small for the heads and converter. Also my opinion the 1 5/8 headers seem small for the 215cc heads. Holley 750 is good. With your setup on bottom end I would not rev over 5500 rpms.