Volvo 122's usually don't rust badly. 1800s are way worse. That said, I have some 122s on my property that look like they've taken a decades-long bath in a flooded salt mine.
As for our fuel cell...well, you see, back in the Tunachuckers 122's day...the Lemons rules on "safety" and "fuel cells" were a bit more lax. That said, we absolutely had to ditch the stock tank- the fuel filler stuck out about even with the back bumper. It would be worse than a Pinto. One of our teammates found about a 15 gallon rotomolded plastic "fuel cell" from a relative who used to drag race and we removed the stock tank (which sits through the hole in the trunk floor) put some metal straps around it to hold it in place, and called it good. Hey, it had foam inside it...
Towards the end of the car's career Lemons started requiring a metal encased fuel cell. So, I welded up a box out of ~16 gauge sheet metal and we dropped the old plastic "cell" into it, and then back into the trunk hole. Again, metal banding straps held it in place.
For reference, our driver's seat was some sort of plastic/ fiberglass thing that I wouldn't put on a lawn tractor now. But again, back in those days you could still run a stock driver's seat, so it was sort-of an improvement. Maybe.
What would I do now? Well, it's a little tricky. Obviously you want the fuel cell as far forward and low in the trunk as possible for weight distribution, but the rear axle kinda gets in the way. I'd probably take a stock tank and cut it in half horizontally, and leave the bottom half in the car to act as a "floor" for the cell. Then get whatever modern, safe fuel cell that fits in that size hole and weld angle iron around it to hold it in place. A 5/16 fuel line should be plenty to feed any B20 you care to use.
Can you tell I've been thinking about this for myself? ;-)
And, yes, I have a warehouse full of used Amazon parts if you need anything- I ship almost anything, almost anywhere. For new stuff, obviously you probably know about IPD and Olaf Malberg and VLVWorld.
Tunachuckers: 15 Years of Effluency
'08 - '10: 1966 Volvo 122, "Charlie"
'10 - '18: 1975 Ford LTD Landau --> 2018 - current: Converted into 1950 "Plymford"
'22 - current: 1967 Volvo 122, "Charlie ]["