There are a couple of issues, like I haven't found a 5 spd solution to the 75 and older trucks. The 4x4 5 Spd is longer by a fair bit and requires some fab work on the trans mount and driveshaft shortening. The 2WD 5spds also can have a different spline count on the trans yoke, so you might need to swap parts there too.
Overall it is not a major project to swap, and it has been covered in much more detail. Search for 5spd trans and you should find a bunch more info.
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95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
