Cut the top off yours and solder it to the bottom of the good early one.

Cut 1" down the side from the top, cut the early one 1/4" from the top, tin the areas that lap + a little further to give you a place to allow the solder to run in easier. I'd try sanding and tinning any spots inside the tank if needed, BUT,I'm not sure how that would go on the original lead coating.(I don't know if there is a lead coat inside, I've never looked and suspect Volvo might have done better). If you do this method, you could jack the top up higher for a bigger capacity if you make different cuts and extend the float arm.

Alternately, if the strengthing ribs on the top are the same, you could cut a hole around the sender section, making sure that you have 1/2" overlap for solder.

If you have to buy a tank somewhere, get an injection tank, they have a 1/2 inch pickup with a strainer. I used one in my car, the 1/2" pickup supplied my electric Carter, the 5/16" return, I used to feed the mechanical pump. The return on the E tank dips deeply in to the fuel and was no problem when running down the highway with the electric pump off.