Hi Tom - some issues you'll run into with the GMC 302 into the AD Chevy.
(1) front crossmember - yes ,as you said, you'll have to install the GMC cross member into the Chevy.
(2) I believe the 302 block is about 2" longer than the 261.
So, you'll have to move the radiator forward, considerably. Not a difficult task if you have fabricating skills.
(3) In this hobby, I don't think that economy is as major an issue as it is for the everyday driver that you put on 15K+ miles a year.
Therefore I'd think about which engine (H.P./ torque) would be best suited for your needs, as opposed to the cost of gas.
Good luck,
Whitman
