Nv3500 vs ax15

T5 out of an early S10 is not good enough

you need a world class unit really and one with quite a long input
and a good supply of 9.5 10 or 10.5 inch clutch plates

T5 out of a 2000-2004 mustang might be robust enough (long toploader type input)
Z spec racing T5 might be (short toploader type input)
V8 mustang T5 might be (some long some short)

the only bit of S10 T5 you need for a truck or bench seat install is the tail housing and shifter.

couple of caveats

late model world class S10 T5 tail uses an electronic speedo but will probably fit the mustang T5, both are world class. The electronic pickup will be in the wrong place so you'd need to come up with a way of mounting your chosen reluctor at the new spot on the output shaft..

early model s10 tail needs work to fit the tail housing onto the late model worldclass mustang t5
it has a mechanical speedo hole. it is in the wrong place for a mustang output shaft, so you need a kit or some ingenuity to use a split collar and some shims and a modified chevy speedo gear to match with the chevy bullet speedo drive you need to use at a place on the output shaft that was not designed to have the drive gear mounted on it.
the housing needs the scroll pattern at the end of the 5th gear area to be machined flat, level with the lowest profile to convert the housing from none world class to world class.
If the Mustang Trans is the type with 1) a Bolt 2) a reverse light switch and 3) some strange metal stud in the drivers side. Then the end of the 5th/reverse rod is too long for the pocket in the earlier none world class tail housing, swap for earlier rod and spring or cut/grind off, just enough, of the end in order to get into reverse. re counter sink the oil hole in its end and leave enough of the fat end of it, to keep it aligned. if there are only two things bolted into the side of the trans case no weird pressed in dark metal stud (supports the later spring/revers/fifth rod) you are ok

if the 5th reverse brake has a tab sticking out of its retainer use a flat milling bit to make a hole of 1-2 mm deep at the top of the scroll section behind the retainer on the back of 5th to trap that tab and stop the retainer spinning so it acts as a synchro/brake for going into reverse.
or snap/grind the tap off and just make sure you pause in neutral before going into reverse like you had to with all T5s before 2000 and all since about 2005.
this Tab/brake thing was not continued as it never worked very well.

pictures here https://www.jalopyjournal.com/forum/threads/t5-transmission-s10-tail-extension.1247081/

the s10 housing will put the shifter box just in front of the deep part of the cross member in the tunnel if you use a mopar bell and a 3/4 inch adapter. Using a thinner 1/4 inch steel adapter leads to easier fit and less trimming of the front of the cross member becase the trans and shifter are 1/4 to 1/2 inch further forward. i.e you just cut single skinned floor tunnel not the double skin at the edge of the cross member. basically to fit you will need to trim off the front upper most edge of the cross member but not the main Meat of it. as it were.

On a small tunnel car the drivers side of the box upper edge will hit the tunnel. air hammer the tunnel or cut to fit. this is a problem on 60s auto floor pan but not on duster floor pan

i got this far but then chickened out on a car that cost me 11K and is now worth 6-8 x that...
yes i'm a chicken.... i stuck my Australian Borgwarner single rail back in (like a ford SROD but smaller and with sensible ratios and a dip in the shift rail for the cross member) this is the Aussie small block 4 speed used my ford chrysler and leyland downunder no 833 for them....

you will need the chevy mount not the mustang one the chevy mount goes on with m10 1.5 bolts on an S10 housing/mount/platform