If the motor runs right with the firing order of 1-3-7-2-6-5-4-8, does not mean that it is a 351. A 302 cam will fit in a 351 block. The 302 firing order was 1-5-4-2-6-3-7-8 up until 85. Then, when they introduced the HO's they went with the 351 firing order. The easiest way to identify a 351 from a 302 is to measure the intake between the heads. Or look at the block where the timing cover bolts to it right next to the head. If the head/deck is directly above the bolt, it is a 302 block. If there is about 1 inch of space between the bolt and the head/deck, then it is a 351. I hope I have shed some light instead of confusing you.
The above information simply: Measure the intake between the heads and determine the firing order. Remember the distributor rotates ccw. Post the results.
FWIW, it is a 78 block you are working with.