Something like this would keep things tight though you'd have to relieve the 5 ton center hole for the deuce studs, then chamfer them and the adapter as a unit for the deuce lug nuts to seat. You'd use 8 lugs to hold the 5 ton wheel to the adapter.
Keep in mind that the deuce wheels are lug-centric. You would still need to have the lug nuts in a chamfer somewhere to keep that plate centered. Using your idea of counterboring the holes and puttin the studs in backward so no 5 ton wheel mod would be needed *should* only require the rear of the hubs 6 hole pattern to be countersunk for the nuts to seat and center the plate. Then, as you've photo shopped, all 10 studs could be used. Plate would need to be prolly in the area of 3/4"-1" thick to make sure it had the meat enought like a stock hub mouting flange.