you can always get a marine control box for just about every diesel engine thats placed in a class 8 truck. dont get caught with it in it. and it will surely void any warranty. i guy i used to work for had cat do some tweaking to his c-16 which stock was 600. after some work to timing and drilling out the injectors they got it to 829. then some more programing and last i heard it was 930. with a marine box for it the cat guys were projecting it over 1300. i dont know if the drive line can handle that much.
alot of companies make a 53' flatbed. I myself have only seem one 53' flatbed with a spread axle. and it was a 10'2" spread. and the axle is measured from center hub to center hub. it might seem over whelming to scratch build. but after you start getting into it and do your research on what you want to make. its just a walk in the park. its also alot cheaper to build your own.
try going to your local tech school that offers machine shop that has a c& c machine. talk to the principal about that. my local area tech school did a set for me for cost of the material. after all the teacher was teaching " how to do it" . just an idea.
fact is you should build to build cause you enjoy it. you show off your builds cause youre proud of them. its not to say you dont build nice things or that your builds are not as nice as someone elses. people like somethings better than others. just cause certain people get 100 replys to their post and others only get 1 or 10 dont mean your build isnt good. its your right to post and be proud of your builds. others may or may not like them. again that dont mean your builds are not good. they just may not tickle someone elses fancy. everyone that posts their builds should be ready for positive and negative feedback. its just a fact of life. the important thing is you build your builds the way you like. if you like the final outcome of your build then thats all that should really matter. its a hobby for your enjoyment.