As others have mentioned, the rear cab chassis tank is probably your best bet. Perhaps this will be of help. It is an older press release about an aftermarket metal rear 55-gal tank, and also has OEM part numbers for the gas and diesel cutaway Express tanks it replaces: https://www.radiatorexpress.com/blog/post/2020/05/19/product-announcement-2004-207-express-savana-55-gallon-rear-fuel-tank
The 50 gal fuel tank for box truck hangs behind rear axle. Vans don't have nearly the rear overhang of the box truck, so the 50 gal tank isn't going to fit. But removing the spare does open up some space between rear axle & receiver hitch.
In similar applications, the tank from 85-95 Chevy S10 Blazer is a useful rear mounted range extender. It's cheap, readily available , 20 gals, has built in fuel pump & sender, Size 33"W x22" F-R.
I've put a 33 gal plastic replacement tanks for a OBS bronco in place of the spare, in a couple trucks. I'm just about positive it'll fit in the chebby. Just don't tell anybody.
Not the cheapest option, but this might provide some inspiration:
https://titanfueltanks.com/products/spare-tire-tanks/titan-spare-tire-auxiliary-fuel-system-for-gm-4010211
IIRC, Titan's lack of support for gasoline isn't a technical issue--it's a regulatory hurdle they can't cross. If you're willing to build your own transfer hose I bet this would work perfectly.
In reply to Tom Suddard :
Or, for $99.00,
minor mods to a ferd bronco tank mount straps...
I guess "throw money at it does solve some problems, but I've always had to go the cheap way.