i work on 2stroke detroits for a living on boats.
mostly 149 series, but a good many 71s also.
there are 71 series engines that lay on their side. thus only being 34inches tall 41 wide.
Most detroits are long enough that youd have to do some serious damage to the firewall. Seeing how our shipyard have dozens of 71 series laying around I've wanted to put a 6v71 in a truck for awhile. You'd need an f450 and some heavy duty reinforcement.... 3500lb engine is obviously an issue.
I think the biggest issue is indeed the weight.
I mean, a 4v53 is great, but you're looking at not very much horsepower for what the engine weights.
people that know how to work with the gov and rack are getting fewer so familiarize yourself, they're really simple engines.
but if you can find a way to deal with the weight, and are willing to live with a cylinder or two being in the cab with you, it'd be awesome.