Post some photos of the front part of the block and we can tell you if it's a Windsor or Cleveland.
Since you don't have the original engine this won't be a "concours level" car. If I were in the market for a non-concours '70 Mach 1 I would prefer a well built engine with adjustable rockers and pushrod guideplates exactly as the heads you bought have been set up.
A 351C 4V will blow through 300 hp on the dyno without even breaking a sweat.
A 351C 4V in my 66 mustang.