My trailer frame is actually the back half of an FJ55 Land Cruiser, not because it was what I needed or wanted, because its what I had to choose from. I could have gone with an FJ40 frame for ease of mounting to the tub but then I had to salvage a good FJ40 frame... at the time we had a stack of FJ55 frames sitting there so the decision we pretty easy.
Would I do it again? Well, sorta. I was in a hurry to get it mobile. It was approaching the winter of 2001 and I needed to be able to move parts with it. I could have fabbed up a quick and dirty utility trailer (or just bought a $200 used utility trailer) but I had the idea in the back of my mind for an off-road trailer and so this could temporarily kill two birds with one stone. I was also just finishing my FJ40 so time and money were short... I literally made this frame in a couple of hours, the leaf springs & shackles were set and in place. Some minor provisions for the tub mounting and I was set. It would have taken FAR more time to build an actual frame from scratch, even with our full fab facility 8 hours is more realistic by the time you had it to where I was in just a few hours.
Fast forward. I've been toying with updating my trailer. Its getting pretty hammered, has been brushed up against rocks and sat outside through 8 years of Utah's cold winters and hot summers. The paint is beat, the frame is pitted from a constant barrage of rocks and my needs have changed a bit over the years. I've thought about starting 100% from scratch, but I really like the FJ40 look and given that this tub is actuall fiberglass it is amazingly light, my whole trailer is under 500 lbs as setup. Much of that weight is likely the way overkill frame, so if I keep the tub I will likely go with a much more proportionate frame with cleaner mounting and a lower profile.
What will I gain? Honestly in my setup I would be really hard pressed to save even 50 lbs with a completely fresh frame, that is alot of time & work for a 50 lb savings. The functionalitly of the trailer will not change a bit with the upgraded frame, there is really nothing lacking now. Aesthetics is where I stand to gain the most... for some that ranks pretty high, I'll admit that I wished it were a cleaner look and thus that is a major motivating factor for a complete overhaul with a new frame.
I've seen the frame used on FJ40's, YJ's, CJ's, Samurai's etc... and really all of them work as good as could be expected? If you have the tub already mated to the frame, you'll be hundreds of dollars saved and dozens of hours ahead by salvaging some form of the stock frame.