Alex,
Where is the tank you have now and is this a short or long bed?
Short beds (style & flare sides) used a 16 gallon side tank.
Long bed style side used a 19 gallon side tank.
Style side beds, short & long, used a 19 gallon rear tank. (flare side did not have a rear tank)
Are you looking for the measurements for a 19 gallon side tank?
I have one in storage I could get them from.
The 16 (smaller) next to the 19 (larger) tanks
You also did not say what year / motor truck you have and if no rear tank you could add a rear tank using factory parts for the tank (straps, cross member, hardware, filler hose / neck) and then would need to graft the door into the body.
Depending on the year / motor you could use a under dash switch to switch between tanks & senders.
The in dash switch you might be able to bolt in place but the wiring I think would be harder to do if placed there. Dual tank trucks used a different wiring harness, main & frame.
If there is a will there is a way
Yes I have added a rear tank using all factory parts (harness & dash switch) making my flare side a dual tank truck that was never an option.
Dave ----