I need advice!

[truckster]

I've always planned a sb400 for my 70 3/4 ton; I even have the vortec heads and manifold and the headers sitting in my garage. Today someone offered me a 455 Olds with tranny for a really decent price. My problem is this: Do I go with max cubes and forget about mileage, or do I go for the sb and sacrifice some displacement to get better mileage? My wife is OK with the bb, but I know I'd have to fabricate some mounts and maybe massage the firewall to get it in there. I'd probably just put a comp cam in the 455, while I was planning to put more performance parts into the 400. I don't drive very far to work so it won't matter too much if it's my daily driver.