loose bolt on main gear box to bellhousing

One of the four bolts that holds the main gear box to the bellhousing on my 1967 SWB Series 2a petrol will not tighten up. Nor will it come back out. Simply spins in the hole. It is the bolt on the bottom left corner of the gear box case and since I can’t torque it down I’m getting way more drippage than normal from the gear box. Based on the diagrams I’ve found, the bell housing is not tapped. These bolts simply screw into nuts on the inside of the bellhousing. If that is correct the bolt may not be stripped, just too loose to begin torquing. Just looking for confirmation that is correct. If so, it would seem as if the only solution is to pull the bellhousing loose to access the nut, and tighten up the bolt for main gearbox case, then reassemble?? Big job for one loose bolt! I've tried sliding a thin blade under the bolt head, and torquing the bolt at an angle to get some pressure on the nut to stop the spinning, but so far nothing has worked. Thanks for any suggestions.

Yes, those bolts have nuts on the inside. I have had success using an impact to tighten a loose one. Just start on a low setting.

jac04, your suggestion to use the impact wrench worked. It wouldn't torque up completely, but may be enough to at least solve that one leak. Thanks!