Not to be an idiot...but whats a synchronized transmission mean
All manual transmissions have synchronizers to help the moving components come to the same speeds as you shift through the gears. As you accelerate and shift through the gears, the input shaft is raising and lowering in rpms with the engine while the output shaft matches the axles and continues to get faster as you accelerate through the gears. The synchronizers and blocking rings are what helps those components come to the same speed while making the shifts.
What Stumpbumper is referring to are the transmissions with the 6+:1 granny first gears where they are not synchronized. What this basically means is that you must be stopped to shift into the granny 1st gear, all other gears are synchronized so the shifts can be made while moving.
As Cutlass and CSP mentioned, all of the 3 and 4 speed trans have the same 1:1 top gear so there will be no difference in highway driving. So when your choosing between 3 and 4 speed transmissions, you are basically choosing the overall strength and which 1st gear you are looking for. The only time I would pick a T19 is if I were running over sized tires with too high an axle ratio and was mainly being driven on the street. In that instance the 4.03:1 first would be the most usable for starting on the street. The 5.11:1 would be too short and the 6.32:1 is a granny. So either you're looking for a granny first gear or your not.
I traded out of a T18 to a NP435 because it was a direct bolt on to the Ford engine and has a steeper 6.68:1 granny and steeper 3.34:1 2nd gear as compared to the 6.32:1 and 3.09:1 2nd gear of the T18/19. The granny gives me a 67:1 final crawl in 4Low and the 3:34:1 2nd gear is 8% lower than the T18/19 which is better for starting on the street. 2nd gear in 4Low has plenty of nuts with the 302.
T18 T19 and 435 are all brothers. They all need the adapter! He already had a T18 so he was able to use what he had!
The T19 in the 4-1 and 5-1 synchro first allows you to down shift to first while moving. I had a T19 I put in a trail rig (74 cj5) and loved the ability to grab a lower gear while moving down the trail!
I trail road and wasn't rock crawling, so any lower first would have been un-usable for me but everyone needs to set the jeep up the best for them!
Actually the splined part of the two shafts are the same it's just that the pilot bearing nose of the jeep shaft is one inch longer. It may all fit in the crank if you use the right pilot bearing or you could maybe just cut one inch off the nose of the jeep shaft. I just pulled the 351-t18 set up out of my jeep to swap a 401 in. I have a lakewood bellhousing and the Advance Adapters motor mounts for it for sale but the shipping would be cost prohibitive as they weigh at least 50lbs.