The Mustang transmission will not just bolt up.

The Lincoln flathead is fairly heavy. So are Chrysler Hemi motors, and most big block V8s. I don't think the weight will make your truck super hard to steer. You will need to keep the stock large diameter steering wheel. As with any manual steering, expect it to be heavy at very slow speeds.

I doubt low power will be a problem with a Lincoln flathead. One of the 48-56 truck contributors, Chuck of Chuck's Trucks, has a 337 in a 2 1/2 ton 4X4 (the pictures are impressive, thread here). He reports more power with his 337 than he gets with his supercharged Mercury flathead.

The Lincoln flathead will definitely weigh more, and cost more, than a 302. On the other hand, it will be vastly more interesting and hopefully satisfying. I wouldn't walk across the parking lot to see another flathead era Ford with a small bock V8. I've seen enough to satisfy me already. I'd definitely go check out a 337 powered vehicle.

*disclaimer* - I don't have a running 337 powered vehicle. I have an F-7, an F-2, and a spare Lincoln engine & transmission. I plan to put a 337 into the F-2 using F-7 parts. The plan is to make the 337 powered F-2 look so factory that even most "experts" will have to ask if it was an option (it wasn't).

 

I will keep looking for a transmission for the 337. I have a few months before I can really do anything anyways, as I'm a football coach.

I figure I can always find a 302 so I'll keep searching for the next few months. Hopefully something will turn up.