[~The Jester~]

Bottom line: ALL 351w and 302HO motors use 1-3-7-2-6-5-4-8 period.

If you take an older 302 which uses the 1-5 firing order, and swap in a 351 cam, it'll use the 1-3 firing order. That's exactly what I did, used a 1980 truck 302, 351 cam (and firing order), and some other goodies and had it going 11.71 at 118MPH. Your backfire is caused by something else, not the firing order. You got that part correct.

As far as the heads go, a 302 won't "bolt on" to a 351 block. ALL 302's use 7/16" head bolts, 351's use 1/2". Have a machine shop drill out the holes and you'll be fine. However, I personally would NEVER EVER run one 302 head, and one 351. Heads HAVE to match. If you're gonna change them, change them both.

I have a set of 1969 351w heads that need gone over, but aren't cracked or otherwise trashed. I'm a union firefighter, so if you're using them on an FD boat, I'll be happy to help out. Let me know on here, or PM me.