Gas tank options on a bagged 82 ?

[NZSquare]

Hi all,
Ive got a bagged 82 wrongbed c10 and just finished up the bridge etc out back (see pics). Its all worked well but my saddle tank is now too close to the deck for comfort so I want to go to the rear mount underbed type.

Im in New Zealand so shipping a performance online one isn't really an option (450 + 200 shipping) and they are 10 1/2 " deep.

Conditions are :
-fit within rails (26 x 22 x 8 deep approx)
-use factory fuel gauge.

Im open to suggestions, searching showed loads of guys putting mustang and burb ones in 67-72's and earlier, but what about the newer ones ?

Ive found RCI and summit fuel cells that fit but am not too keen on plastic.

Any suggestions are appreciated, right now I'm looking at having one made.