Hmm.. I'd say if this is something you really REALLY want to try, then you might try finding specifically the one-year 1985 Honda TRX250 (NOT the R) FRONT rims, which are a 10x5.5 4+1.5 offset at 4/130. These should fit the rear of FL350/400 and likely put each rear at about 5" narrower. That might clear you for the trails you are after?
Your best option then for tires on these rims would be a 23x8x10 +/-1" or there about. I'd recommend ITP Mudlights in this size as these are front tires but are nearly identical to Mudlite rears and are quite beefy. That would be the only tire I would recommend you run on '85 TRX250 front wheels for the rear of your machine.
LOL btw
DWT has an aftermarket wheel option: 009-07 is the part number... The problem though is they are Blue labels (EWWW) and would likely bent up very quickly in the rear of an FL; plus they're offset slightly different than the OE TRX250... Hence I'd recommend you try and find the Honda OE front rims which are STEEL and stronger for heavy machines.
Here is a used set for $90 on ebay:
https://www.ebay.com/i/293185059763?chn ... gLGOPD_BwEGood luck. You are in uncharted waters; but if its a place you'd ride a bunch then it might make sense to have this set available to you?