there are two ways of telling if a puppy has 3.42 gears
they will say MPG+ on the rear of the cab, behind the doors, at the top of the bed rail hight. letters "about" 1 to 3" high
the other way
and its not 100% of the time is they have a 4 speed not a 5 speed, unless it was sold in CA state.
then for some odd reson its a 5 speed
also if a gasser has a 4 speed, i think its a 3.42 as well, but im not 100% on that.
a 3.73 will be much easyer to find sence there most commen
HO yea theres the auto trans puppys there all 4.11s
ive yet to find a 3.90 gear in any pup, mabe its a pre 1981 LUV thing
do you live in a hilly area? or have a drive way that you stop at the top to look for traffic or move lots of cargoLBs

if so you will like the 3.73 better.
P.S.
if you find a pup that still has the diesel trans, that trans / bell err frunt half of trans case,
is very hard to find. the starter is on the other side of a gas engine.
thus to change a LUV\PUP to diesel you need a engine and bell housing not just a engine
you may find a buyer for it.
if you where close to me i whould give $60 in a hart beat to get one and thats just the bell.