I moved the spare on my last 5er, to under slung, with a winch setup. The ease of the install mostly depends on the access to the 5er's frame. I got off very lucky as the spare location was optional and the frame brackets were already welded in place. Bolted on the winch and located the crankshaft, done in 2 hrs.
Look at 5ers on sales lots for units with the under slung spare, for ideas for location and attachments.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.