Is it possible to bolt up a 7.8 diesel in L800 truck that currently has a 5 speed and a 429.

 

No, don't bother. Cab is different, front frame crossmember is different, bell housing bolt patterns are different, not to mention fuel system and electrical. The 7.8L was a Brazillian built boat anchor anyway, and they are getting hard to find parts for.

 

They are far from a boat anchor. That honor goes to a Detroit 8.2.
Better off getting a truck that already has the engine you want. MD trucks are pretty cheap these days.

 

I had a couple. Distinct nautical flavor, that's for sure! Actually I had pretty good luck with mine from a dependability standpoint, but the 6.6L and 7.8L are very large and heavy for the amount of power they make. In 1993 Ford dropped the 6.6L and 7.8L, replaced them with Cummins B and C series diesels. Still have a 1994 F-700 with a 230 h.p. Cummins B5.9, and it outran the 1991 7.8L we had with ease. 8.2L Detroit? Had them too! Probably the single most economical medium duty truck we ever owned was a 1985 GMC 6000 that had a 165 h.p. naturally aspirated 8.2L. Never broke, used very little fuel. Did I mention that it was slow? Now, any of the higher h.p. turbo 8.2L's like the 225 h.p. were trouble. Couldn't keep head gaskets in them. Take one apart and you can see why. Anyway, like the post says, you are a lot better off finding a Ford built with a 7.8L than you are trying to install one in a truck that didn't, particularly a gas job.

 

Here's a 7.8 in an 87 F700. I did the fuel screw, 3 clicks tighter on the gov springs, and strait pipe. Advancing the timing should have been done, but I never got around to it. It had 2 names that the guys called it; little rocket, or old smokey(when I took out the fuel screw for a while).

YouTube - Ford 7.8 straight piped F700