66 air gas tank and motor mounts

I was wondering what some people thought of for aftermarket air systems for a 66 GMC pickup. Classic seems to be the only one. Vintage has for Chevy but not Gmc. Also, the previous owner swapped a 350 for the six and I'm wondering if I should move the mounts to the forward holes? I read another post about firewall clearances but am thinking about header clearance too. There is a lot of room up front but I want to put a MSD HEI distributor in. I'm also looking for a rear gas tank set-up. Any ideas? I'm leaning towards a plastic fuel cell type. Any thoughts or advice are appreciated.