The NSG-370 from a TJ should bolt right in. Just make sure its not from a KJ as the KJ has a different bolt pattern than the TJ and the bellhousing is integrated into the trans.
No need for all that, same bolt pattern and spline count.
The gearing is a little different, but IMO the final drive sucks for me. Also from what I can tell the NSG-370 is a little weaker than the NV3550, but is on par with the AX-15, though I don't have any reference information available to support that.
NV3550 Gear Ratios
First Gear 4.01
Second Gear 2.33
Third Gear 1.39
Fourth Gear 1.00
Fifth Gear 0.78
Reverse 3.57
NSG-370 Gear Ratios
1st - 4.46
2nd - 2.61
3rd - 1.72
4th - 1.25
5th - 1
6th - .84
Reverse - 4.06