351w block questions

I thought they were using the 351M past 1979.

Its a modified version of the 351C. I am still reading up on them, since my friend has 2.

Trust me you don't want a 351M.
The 351 Windsor Block to get is the 1969 block. 69' Block's are beefier and have very thick main caps and can handle 700HP with a girdle without any problem's whatsoever. One downside to these block's is that they use a 2pce rear seal which is really a minor set back. The 78' you are looking at is really just a plain old Windsor although having a Standard Bore block laying around is kind of nice. Later Windsor's (I forget which year) were roller cam compatable right out of the factory which is kind of handy, although eariler block's such as the '69 can easily be retro-fitted so a roller cam can be used. Basicaly if you can find a 69' Block, snag it, if not later block's are still good and can still handle alot of power, especially when a main cap/stud girdle is used.

351M/400M's aren't really considered much of a performance engine. They were kind of a knock off of the 351C exept they used pos/smog heads and low compression. They are a fairly heavy & bulky engine especially when compared to a Windsor. Basically there is nothing similar when comparing a Windsor to a Modified. You can tell the difference just by looking at them. These 351M/400 engines found themselves in truck's and big car's. Production of these motor's ended in the early 80's.
351C's are a good engine to have but don't waste your time with Modified's.

the 74's and back were stronger to a certain extent

yep 69-70 are said to be the stronger stock blocks, 9.48" deck height
71-74 are known to still be stronger then late model blocks since they still have beefier mains and webbing 9.50" deck height.