Just picked up a neglected 78 F150 last night for pretty cheap. It has a 351M, and a C6 tranny. Both the motor and tranny are in need of rebuilds.
The rig may become a tow rig and needs to handle my toyota rockcrawler / trailer.

What performance mods do you Ford guys recommend?
Looking at cams, heads, full rebuild kits, etc....Dont really want to spend too much $$$$ since I will rarely drive it. Just would like some good torque and reliablity.
I've also browsed the Summit catalog and there arent many kits mentioned for the 351M. What model do I follow when i order parts?? 351C?

Or just save my $$$, part the thing out, and go buy a real tow rig.?

I'm not a fan of the 351M. What I'd do if I wanted to use this vehicle is sell off the engine, have the trans rebuilt and spend the money I would have on the 351M on finding a rebuildable or running 460. This will bolt up to your trans that you have now. You'll need to modify or make some motor mounts, I'm not sure on the radiator. The best thing about a 351M is it shares the same bolt pattern as a 429/460. But if you don't want to sink some dough in this thing just look out for you another 351M they are out there, everybodys pulling them out to replace with something bigger and better :D you can proably find a heckuva deal on one.

A bigger motor is always good:D .
Any other swapable motors? chevy, etc...?

I'd keep the 351 block, but pick up a set of 400 pistons, and a 400 crankshaft. Will make a much better tow rig, and provide good torque. Cost should be substantially cheaper then going to a large motor.

The ford 351m, and ford 400 are identical engines, with the only difference being the crank shaft and pistons.

Sell the 351m and buy a 460, no Question about it.....