i'm building a 302 with a 351 camshaft. i'm told that this works just by changing the firing order at the dist. what i'm having difficulty understanding is how this works with the timing of the "throws" on the crank

 

Just wondering... why are you putting a "351" cam in a 302? I think what is important is what the specs of that cam are for your specific 302. Older 351 cams were basically all bottom end and were done at about 4500 RPM. On newer EFI engines the 351 cam again is basically for better bottom end torque and will yield less top end horsepower on a newer 302.
The firing order of earlier SBs were the same for all mills:
1-5-4-6-2-3-7-8.
I believe the firing order changed for both in around 1985, but not really sure.
Either way, the cranks are the same so the "throw" as you refer are the same for all of them. The cam itself will determine the firing order.