Considering buying doublewide moble home that has been stripped, including the breaker box, feeds from the floor and has been cut down to within about 1 ft from floor, can I slpice and set another box that will be up to code for resell, or might use for rental and don't wan't it to burn down, considering the 3-m splicer kits

I would use a 4 x4 or 6 x6 wire trough at the bottom of the wall fit between the studs, and have all cables enter using proper connectors, and do all the spicing in the trough.

Then you can use two or three 2 in PVC conduits to rise from the trough into the bottom of the panel to extend the new wire spliced in trough.