awesome member budnikguy hooked me up recently with this high capacity suburban gas tank.. thanks Adam!
I've seen bits and pieces of info on these in older threads and wanted to share the actual size..
measures out to 50 gallons 
they are reportedly factory or dealer option, Adam told me he's actually seen a few of them on burb's he's acquired..
it's not the double hump I've seen more pictures of so maybe it's custom, less common, specific dealer or year or ???
my objective is to install it into my '71 K20 burb if it will fit in the 4x4 chassis with rear exit dual exhaust and receiver hitch..
fuel sender appears to be the standard 5 bolt stewart warner style..
matching up the proper resistance for the fuel gauge and tank depth, 14" in this case, should work fine..
I have installed several of these senders in purpose built heavy duty trucks with non-standard fuel tanks..
follow the directions for taking measurements to determine float travel and trim the rod accordingly..
here's a summit page with a few listed;
Stewart Warner Gauge Sending Units
this one seems to have more than enough fitting attachments to run the charcoal canister and a return line if needed..
when I have the time to get into it, I'd like to pull the sender and investigate the inside, see if it has proper internals intact..
I'm a little concerned it only seems to have the one mounting strap, probably want to fab additional mountings..
and since I plan to do some mild off-roading with the K20, might want to consider adding a skid plate under it..
anyone with more information on these, especially in a 4x4 chassis, please chime in, thanks in advance!