welcome to the bolt

51 being an AD truck may have some variations but i am putting a 261 in my 46 1/2 ton. check w/ the AD guys. in a 46 i am using a water pump adaptor plate, look for pre68dave on ebay, very resonable and a nice piece of machining. this lets you place the water pump of the 261 back in the original 216 height location to work through the radiator better. a 53-55 watr pump #3836084 will solve the pump shaft length question. you can also get an after matrket version of this pump from the major flaps. the 261 could be offset or front mounted. in my 46 i am using a 235 front mount that may or may not have to be redrilled to meet the front crossmember. locating the remote oil filter is also another consideration. as of now these are my main considerations.

search the engine section and use 261 as a key word and you will find a ton more info. here are some more places to look.

http://www.inliners.org/tech/261_build.htm
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=491452#Post491452

hope this helped

big ed

"it's only old if you can't find a use for it; otherwise it's cool and i'll use it."