Don't expect wonders from the old inline 6. The compression is low and the chambers are pretty old-school, so efficiency is very poor by today's standards. A modern-headed 302 would probably get better mileage than the old 6 if done right.
But, a Maverick is a well-kept secret. They're not a very stiff chassis, but some careful tubing and subframe connecting can really help, and their best attribute is that they're pretty light. I had a 74 that I was going to put a Subaru STI driveline in. Everyone told me that the turbo 4 would be a terrible match for the heavy Maverick, but little did they know that they weighed almost exactly the same. Even the stock wheezer 302 in mine was remarkably quick. My plan was to do big fender flares, slam it, and do it trans-am style for road racing.
Build yourself a 400-hp 302 and just try to get traction without tubs