I think the 4.3 is 262 c.i., so your system has the capacity to fuel an engine 12 cubic inches bigger than the 250. Sounds close enough to me, in my vast ignorance. And the 305 c.i. system I have can supply 13 cubic inches bigger than my 292 (although mine is bored .060, giving a total of 298 or 299, I think - haven't done the math.) Also sounds close enough to me, in my vast ignorance. Your PROM, being from a 6 cylinder, seems to be what I would need to replace my 8 cylinder one with, though I don't even know if the PROM cares how many cylinders it's running, maybe just the displacement matters to it, and the distributor reports the number of cylinders. Know anything about that? Is there a number on your PROM that you can see, without much trouble?