Offroad-Express

Wrecked my steering box in the safari this weekend, leaking fluid everywhere tried removing it but 1 of the 4 studs holding it to the chassis is turning & I can't remove the nut.
I could grind the stud &/or the nut off but I'm then left with only 3 holding it on.
Anybody got any ideas how I'd get another captured stud in chassis? or any advice would be much appreicated.

Got this sorted went & mowed the lawns, cause it wasn't raining, stoped squealing like a girl & had another look & discovered it wasn't a captured stud but infact a through bolt .
prob gonna put a 2hd hand unit in as quoted $450 for reco, but a 20 yr old steering box, how good is it gunna be? am I risking throwing money away on 2nd hand junk or should I fork out for a reco?