1989 F150 Extended Cab Fuel Tank Capacity

I am the original owner of a 1989 Ford XLT Lariat F150 extended cab (it does
not have the long bed).  It has two fuel tanks.  How many gallons of fuel do the
tanks hold?  I want to begin adding Marvel Mystery Oil to the fuel tank but am
unsure how much to add.

I found the old brochure on my truck, but it does not give me a great deal of
information.  This is what I found, but I am unsure of the accuracy for my
particular model:

Fuel Tank Capacity
(gal)

Reg Cab:  SWB Std.  18.2;  LWB Std 19.

Super Cab:  SWB Std. 34.7;  LWB Std 37.2

I am unsure what SWB Std. and LWB Std. mean.  Something about the bed
size??