[MagicRat]

Did you remove this distributor from this engine, or did it come from another source?

If it's from another source, I wonder if your identification of the engine is correct.

First of all, the 351M and the 400 are somewhat different engines, but use the same block. Do you know which one you have? Or, is it possible the engine is a 351-Cleveland?

I ask this because the 351M and the 400 look very similar to a 351 Cleveland, but have a taller deck and a wider intake manifold. I think, (but am not completely sure, ) that the 351M and 400 distributors are longer than a 351C unit.

If your engine ID is correct, the distributor may be from a completely different engine, such as a Ford straight six engine. A 2 inch gap is huge, and is not explainable simply by the distributor not dropping down fully.

Maybe you could whip the distributor out of your '79 F100 and compare it to the one you are trying to install. If it's the same, then you know you need to source another distributor. If thats the case, call a wrecking yard. They should have the Hollander Interchange Manual, which should list what Ford engines used the same distributor as the one you need, then start hunting down the correct part.