If I recall correctly the FC150 was actually built on a CJ5 frame. It should me a matter of measuring and relocating body mounts, but I have never done it. I would find a buddy with an early CJ5 and take some measurements and try to visualize how to mount that body on the CJ frame (ie, radiator, ect).
You are in for a LOT of work. All the FC 150 frames are straped and I believe only the 56-57 narrow gage ones used the CJ 5 spring layout. The front frame horns are different with the Ross box mounted very close to the front. Just about all the FC frames broke in the front spring hanger area, despite having extra straping on the rails so a stock CJ frame is probably a lost cause.
You will be FAR ahead to just find an FC frame and beef it up before you start building. I would do a full box with and extra outboard plate on the front extending back behind the spring hanger about 12". I would start with one of the later outboard spring mount version (post 57) that use the same spring hanger posts as the FC 170's. These also use the same width axles as the wagons and came stock with 44's on both ends (rather than the 25/44 combo)-much more stable than the narrow gage version. Even though the stock front D44 on the post 57's is closed knuckle, it is way better than the 25/27's. At least you will probably not be spitting the spiders out of them.
The FC150's have a cast iron counter balance weight at the rear that I always thought was a real waste. IIRC, it is about 140 lbs of cast iron. If you are doing a build, I would be putting things like the battery and fuel tank in the rear for balance and forgetting packing around that extra weight. You most definately still need the weight balance.
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