I've done that swap on a M105. Gave myself a lower trailer with electric brakes and 8 lug wheels; had the same wheels as my DD, so I always had a spare (or two). There are a few threads in the trailer section about the swap.
I went with a 4" drop, Dexter 6k axle. Spacing was per information I found here (79" IIRC), but it could have been a few inches shorter without concern of rubbing.
I do NOT recommend the Dexter axle though. It was starting to bend after minimal use (3 runs with 3k lbs of water in the bed) and the company stated that I did not follow their recommended spring spacing causing the damage. The springs are at 44" O.C. if I'm remembering the numbers correctly, I can measure the original axle as it's still in the yard. It is NOT easy to relocate those springs, so the axle needs to be designed for the spring spacing as well.
I had a local shop rebend the Dexter axle back into true, then I put some gussets in place. I've sold the trailer to make room in the yard, otherwise I'd get pictures.
HTH