If this truck still has 120 inches (or 10' as you claim) of C/A that is correct for a 16' body length. a couple of feet of AF (frame behind the rear springs) then it is an excellent candidate for a body up to 16' as that allows for U bolting the rear.
I don't know much about the Cummins 5.9 in a TF sereis and it may be quite a chore.
IF it would fit then engine mounts become the first concern after the real first concern of will it accept the trucks transmission. Then there is the radiator issue of course.
And the steering as that Cummins is sure to be lot heavier than whatever is in the 55'now. It might be a lot esier to get a front frame clip off a Dodge so you could install it and the benefit of steering and cooling are a bonus. Also get the tranny and don't forget the fuel tank, lines, filters, intercooling set up, etc.
Do you see where I'm going? I don't want to throw cold water on your enthusiasm but it all sounds so doable when your 13 but becomes a whole lot more difficult when it comes time to do it. It's going to take a great deal of lawn care business to get this done and most all your buddies.
Having said that I wish you all the success in the world and I like nearly everyone here stand ready to help however we can. At least on the TF chassis end of things.
Send for Jim Carter's CD parts disc. Also Brothers Truck Parts, and a great many more will show up on a Google search. All the suppliers do a creditable job. Larger items like cab parts are pretty much from wrecking yards, private owners scrapping one out, etc. Be sure to stay current with our Parts Wanted and Parts for Sale Forums. Likely all you instruments will need to be updated to take care of the Cummins. And lastly, sorry to say, I've only scratched the surface of the problems you'll face. But if your determined then you can make it happen.
Last question...OK? Why do you want or need a diesel engine to haul a couple of lawn mowers and assorted yard tools? Maybe Grandpa's original Chevy engine would (WILL) work as well for you. Think about it.
Go for it!
