can some please tell me the difference betwee the 351m and the 400?

I'd have to do some research to know for sure, but to the best of my knowledge the '73's did not have a 400 available. Most came with the 360, 390 or possibly the 351 Windsor or Cleveland but I don't think the 351's were available until 1978.

To find out do a Yahoo or Google image search of each, then it will be easy to tell, they have obvious differences just by looking at valve covers and exhaust manifolds, air-filter canisters were different., A/C compressors are in different places

its defitenty a 351 M is there anything thats the same between the 351 m and 400?

have you inspected the block to find any numbers/letters (every single block has a code), compare it to the one on the transmission, let us know what they are, so we can tell you if they are matching numbers, and can further determine what you have, that many years it is hard to say it is still factory, but I bought a late 70's Explorer (F-150), from the original owner so let us know what you find...

OK the 351 has a 4" bore and the 400 same, but a longer, 4.00" stroke. The 351 Cleveland has more square valve covers, and the Windsor 's are more rounded off at corners. But if that is what you have , either, it's not factory. Neither engine was used until 1978, before that they came with a 360 or 390, both with wimpy 2bbl carbs and when I bought my 1978 F-250 I bitched about gas being 60 cents, jumping from 48 cents the year before. When you could get it at all.

PS the 351 has a 3.5" stroke, same piston, different crankshaft and connecting- rod