i agree that it is probably a big block ford 370, if youre lucky then it may be a 429. i have a '83 ford bluebird b700 and it has a 370 gasser in it with the at545 allison tranny. to get a liitle more umph out of it you may want to consider changing it over to regular gas. you wont neccesarily see more top end, but you will be able to sustain 55 a little easier than with the current propane setup. to do this youll need to find out what kind of bolt pattern is on your current manifold and then source a carburetor to match or just buy a new intake manifold with the carburetor. the plus to your bus being propane is that they usually stay clean and last longer. the negative, some say, is that propane is harder on the valve train in the engine.
i was suprised to see how clean externally and internally the engine i bought was. i am in the process of getting ready to swap a 429 into my bus. it is a '89 ford 429 that was used with propane. the intake on it has a holley carburetor pattern so all i need for it is a 4 bbl carb and itll be ready to go.
and while jarlaxle is right on the 429 being a direct blot in swap, the 460 with an auto allison is not without a bellhousing that will work. the 385 series of ford big blocks ( 370, 429, and 460 ) while being interchangable in the lighter duty engines are not so in the heavy duty ones. in fact, to my knowledge, they only made the 370 and the 429 in the heavy duty class. i was wanting to do the 460 swap initially but talked myself out of it after talking with some guys on the ford truck forum ( http://www.fordtrucks.com/forums/index.php
). the person whom i talked to said that it was possible with a custom bellhousing to match the 460 up to the allison tranny so i chose the easier route and went with the 429. if you had a manual tranny i do not think you would have this problem. i believe it is allison auto tranny specific.
Good luck with you bus !
TonyC in Iowa