[65crew]

The 2001 frame should work, extending the fenders gives more room to work on the motor etc. You also need to extend the hood or the valance in frount of the windshield.
on mileage, the 6bt gets 20 around town and 22 plus in the highway
the 4bt is 23.5 in town and 24-25 on the road

I would use the 6bt inless you are worried about the weight, it would cost less and as you can see the mileage is close.