T18 is a great trans. Has a great 1st (granny gear) which you pretty much just use offroad. Good for super slow. It's basically a 3 speed with a crawler gear.
Most T19's don't have the awesome 1st (granny gear). Most are like 4.02:1 or something like that. 1st gear is synchronized. Because of 2nd gear being 2.41:1, it is truly a 4 speed and you will always be using 1st gear for normal driving. Try to start out in second and it will do it, but it will be a dog.
For clearance just trim the floor. I can't imagine it needing that much trimming to fit. If you drop the cross member rear drive line angle improves, but at the expense of front driveline angle (worsens). 1" body lift can be helpful in resolving a multitude of clearance issues, but opens up other issues, like will your steering stretch far enough, will your clutch continue to work, now there's a space between bumpers and body. Those are just a few off top of my head, I'm sure there's more.
1978 CJ5 Wheeler
1997 Grand Cherokee Laredo 4.0 all stock daily driver