If you are absolutely sure that it is a 1 to 1 gearbox, & you are not using fifth gear in your current overdrive box, then clean & strip this one & check it for wear & damage, after all, it was in the back of the van for a reason, right? [it is a car gearbox isn't it?]

If all is good, then give it a freshen up with some bearings, synchro rings & small parts as required, & swap it into your car.
Sell the OD box to help defray your costs.
This way you get a fresh & tight box, you get to use all five gears, enjoy the drive, & save a buck on the deal through low purchase price & the sale of the OD box.
Sounds like a bargain to me.