Outer Limits 4x4 Board

i have a gq wagon with a 454 that uses its share of fuel.
I was thinking that if i could move the chassis crossmember that is behind the gearbox forward then i could get a third gas tank under, at the moment there is only 780mm in length and thats a small tank

My question is has anyone done this and are there any issues.

Anything can be done champ, though your best bet would be to go see an engineer first..... especially as it'll be modifying the chassis!! I'd probably go see the engineer that certified the 454 conversion first as he should have details of all the mods for your car on file.... and then take it from there.