I agree with magic mike. If you're already having the 350 built, then you might as well put it in there. It would be the cheapest way too. You could always rebuild the 250 and put it in later, if you ever wanted to go back. I have split headers from Hedman on my 250. I got them from Summit, I think. John

'68 C-10 - 250 with Edelbrock 600cfm 4 barrel, Offy intake, Hedman headers and true duals, HEI, MSD 6A box, relocated gas tank
Soon to have: T-5 tranny