Hey there. Welcome to classicbroncos! Well, I see you've been a member for a couple of months now, but since you just posted figured you still qualified for a welcome-to-the-disease shoutout.
Don't have any info for you on a tank swap (sounds like you're looking for an existing vehicle donor?) but back to the top of the page for others to see.
I believe that more and more people are getting creative in their choice of tanks, but all of us that have specific larger tanks for sale (such as our M1A1 at Wild Horses:
https://www.wildhorses4x4.com/category/Bronco_Steel_Fuel_Tanks) are in that $600-$1,200 range you're initially trying to avoid.
Aero Tanks will make certain setups that are at least partially custom, which is likely what you would need to be able to retrofit stock type pump modules out of later vehicles, while we tend to keep with the early 73-10 ohm range for the Bronco gauges.
While generally a little less expensive for their stock offerings, you'd have to call them about a custom setup and see what they say about pricing.
Depending on where you are, shipping could be an issue too of course.
Where are you by the way?
That's still general custom stuff though, and not from a modern donor vehicle out of the junkyard which is likely going to be your lowest cost up front.
Is that what you're trying to find? An existing donor piece? Or a custom Bronco-specific model?
Good luck!
Paul