If it 'said" 351m/400 on the engine anywhere it has to be the tag on the valve cover since that is the ONLY place the engine data ever resided on these engines. That said, the ssame tag wil indicate what displacement you have. (Assuming, of course, the valve covers are still on the original engine).
On the valve cover tag:
First line is "Engine Family: 351m/400"
Second line "Engine Displacement: 351m (or 400)"
The 351m and 400 are officially small-block 'Cleveland engines', but are distinctly different from the 351C, with taller decks, larger cranks with 351W-sized main journals, and a big-block bell housing pattern. So...they are a small-block that is as tall as a big block and with big-block 429/460 bell housing pattern. Confused now?...yr in good company :toothless