Don't build it over frames, build it over FORMS!
Be sure to glass both the inside and outside. 1/4" strips, double 6 oz in and out and you should be able to walk on it.
Just my personal opinion, but you should paint it too.
BTW, I'm a Jeep guy too, been driving one CJ or another since 1984. I have a soft spot for those FSJ's, especially the big Cherokees. Someday, when I have too much time on my hands, I'll get a full size Cherokee and drop in a modern (semi-modern) diesel drivetrain...Really nice J20 you have there, wanna share any details?
I grew up with pre-chrysler Jeeps as well, my first vehicle was an 85 CJ7, and its been down hill from there. I have a soft spot for the FSJs, especially the J series trucks. I have owned a number over the years, but this is the one that won't leave. If you know your Jeeps, you know that for certain years, the stars just lined up.
Its a 1975 J20, I am the third owner. The original owner used it exclusively as a yard truck for a wrecking yard in WA. He gifted it to a young employee who joined the Marines as a mechanic, and was promptly stationed in CT. He owned it for about 6 years, but never registered it. He proceeded to, as the young are want to do, cut the exhaust off "to make it sound like a Harley", paint all the aluminum and chrome trim black, and put on an edelbrock intake and carb that was way too big for the engine. He was selling it to finance a family vehicle. It turned up on CL about 8 years ago, I was at his house 45 min after he posted it.
Its a 75 J20 with 28,000 original miles (It never left the yard in WA until it went to CT). Factory AMC 401, T18 4 speed, Dana 20 transfer case, HD Dana 60 rear end and HD Dana 44 front end (both have been re-geared to 4.55 and have ABR air lock differentials). There is a small spot of rust on the front drivers side wheel flare, other than that, nothing! Ive rebuilt everything that moves (Engine, T-case, tranny, both axles) and in the process addressed any of the quirky "Jeep" things (Oil pressure to the back cylinders, leaky exhaust manifolds, etc..). I had a custom 6" lift made in Montana. I wanted a 3/4 ton truck to do 3/4 ton truck things, not a super mushy rock crawling suspension. The 6" lets me clear and stuff (if necessary) the 35" tires without cutting the wheel wells or fender flares (which is what makes this thing look good). I kept the Edelbrock intake, but put on an appropriately sized Holly Truck Avenger carb. The entire starting system (which was always a week point on the FSJ) has been entirely replaced and upgraded, along with a new wiring harness. The interior is completely stock, and could almost pass for new. Its one of the few FSJ you will see that everything works on! Im sure im missing things.
The little bit of body work is next on the list, then a mat clear coat (I dig the weather "rat rod" look). I have no desire for it to look new. The black paint will get stripped, along with the spray painted jeep logo on the tailgate. I know a super talented finish guy at the boat restoration yard where I used to work who is going to do the tailgate, repaint the logo white, then weather it to match the rest of the truck.
I toy with the idea of either a diesel or LS swap, but I don't think Ill go that direction with the truck. My wife would like and older FS Cherokee or Wagoneer, so Im keeping my eyes out for the right one, with the idea of swapping in a modern engine, and making it her daily driver.
Its my rebellion against modern everything. It takes work to own one, and just as much work to drive one. No ABS, GPS, sensors, or automatic anything. The security system is the fact that its a big naturally aspirated engine with a manual tranny. Most people anymore couldn't start it, let alone know what to do with a big stick shift. My 13yo daughter summed it up perfectly when she first rode in it "daddy this smells like gasoline, motor oil, and cigars". I said "those are three pretty good things!"
