First off let me say that I hope your purchase went well, and welcome to the Bronco Brotherhood! As for motors. Big blocks are a common retro-fit in these trucks! Just be careful IF you "search this site" for info on the matter! I've been on here a relatively short time and have noticed some misinformation AND some just outright ignorance regarding user related posts'. Then again, I've read some VERY knowledgable folks with good sense and decency who are very learned and trustworthy! Just sayin', trust- but verify! As for the question about Fords' infamous 351/400 M,Mod, Modified, Milford(Plant), Modified Cleveland or what ever it's being called THIS decade, it's a good motor that gets little respect, undeservedly. All of the BIG cruisers,ie. - Police interceptors and the like came with 400's startin' in 1972. I can tell Ya from experience those puppies could incinerate some tires!!! Especially the "old" bias ply tires that came on 'em in those days. Talk about goin' to the wood with it and fryin' the meat off 'em! You'd have to lift after 2nd gear to let the intake belch the carb back out and start all over in 3rd gear! With flat top pistons and "some"resemblance of compression it eats with the best of 'em. What Ford did to "tame" 'em down for insurance AND the related oil embargo of 1973/'74, was ****** the timing(at the timing gear set) and mill off a "dish" into the pistons to further cut down on it's "NEED TO FEED". All it takes to wake it up from it's Ford Factory induced COMA is replace the timing set, give it some carb, some cam,"some" compression and let it breath! But, overall the motor as a group can easliy be souped up and tricked out like any other mill out there. The Big Blocks were NEVER offered in the Bronco's from the factory, in '78 and '79 only the 351m and 400M was offered. The only REAL issue with a Big Block transplant is engine "location" relative to the frame horns. The oil pan HAS to be a rear sump(if you have stock or near stock suspension heights) due to clearance issues with the straight front Dana 44 axle. Also "some" have had exhaust manifold clearance issues with the passenger side frame rail due primarily to using manifolds from a sedan instead of a truck OR van! Just research the internet for motor mounts and other forums that have "tech" boards to get a better understanding of what ALL is involved before comin' to a conclusion on which motor to run. Good luck with it. Again, welcome to the Bronco Brotherhood!