As you probably know, the 351m is identical to the 400 except the pistons and crank. Thus, there are two ways to go.

First would be to find a 400 and just build it. Take your time, as your truck stays in service.

Second is to find a 400 crank for yours.

Everything else is going to change--pistons, cam, bearings of course, valve train. Heads are a question mark, as rebuilding stock heads can cost a good amount, making the move to aftermarket heads easier to stomach.

Everything you need to know is here:

351M/400 Performance Build Up

Bubba's M-Block Ford V8 Workshop

M-Block 351M/400 Parts Reference

And everything you need to get is here:

TMeyer, Inc. Precision Automotive Machining