Some interesting info about the 351M
http://www.projectbronco.com/History/history_of_the_ford_351m.htm
By the way, the 351M and 351C weigh about 550 lbs. The 302 is about 470, 351 W about 510, and the big blocks are in the 640 - 680 range. Springs designed for any 351 would be ok.