I think Northcoast accurately and succinctly summed it up in his post. But I would add more emphasis, as the difference is (in my opinion) dramatic.
I'm eating major crow to say this because it was my snowcat buddy that sold me on the advantages of an automatic over a manual (in a snowcat application). However I think the automatic really is a game-changer. In our Thundercat project we installed an automatic in place of the stock 5-speed transmission and I'm very glad we did.
If I were searching for a Tucker to buy, in addition to the machines overall condition, I would look first for the cab configuration I wanted (2, 3 or 4 door) and then maybe the automatic, and then the track length. The relative rarity what you want in terms of configuration will largely determine the difficulty of finding it, and I haven't even mentioned that little matter of cost! For example if you said "1643 with an automatic and a front blade", I'd say good luck. I think it's easier to swap carriers and tracks than convert a manual machine to an automatic.
That brings up another issue, one that stirs debate on the forum...
When Tucker started offering an automatic transmission option it was the Chrysler Loadflite, a version of the 727 Torqueflite. Both are three speed non-overdrive transmissions. Tucker also offered the Allison AT-545, which is a four speed non-overdrive transmission. I suspect the Allison option was substantially more expensive, but I don't know that with certainty.
Some on this forum, notably Snowtrac Nome and Teamster, are big fans of the Chrysler 727. I think the Allison is the better transmission for the Tucker application. In my Tucker experience I have talked with the factory on several occasions. Most of those conversations have been with Jeff Godard. Jeff now works in sales but he was the manager of the parts and service departments previously. Basically he is an encyclopedia of Tucker knowledge (and a heck of a nice guy!).
The Chrysler Loadflite was offered with either a long or a short tailshaft configuration. Tucker used the short version. Jeff told me of a customer machine brought in for service. They discovered the tailshaft casting was broken and sought a replacement. They could not find one anywhere, at any price.
Others are going to tell you that was a unique experience and the chances of that happening to you are remote. I don't know, maybe they're right, maybe not. But as one who has searched exhaustively for parts (and come up empty) I think that's something to keep in mind.
Sleddogracer, I would suggest you measure the overall length of your 440 and automatic transmission and then compare that to your current setup. I think you'll find it won't fit.
I'm guessing you've got a Chrysler 318 Industrial engine and a four or five speed manual. The engine likely attaches to the Tucker with a casting bolted to the front of the engine and a single stud that bolts to the Tucker. The transmission bellhousing likely has an "ear" on each side of the casting that bolts to welded on brackets on each side of the Tucker. The transmission itself bolts to the bellhousing, but there is no separate transmission mount. Also note the parking brake drum setup on your transmission. Now go look at the 440... I'm guessing everything is different.
On our Thundercat project I talked with a couple of shops that build custom driveshafts and IIRC the shortest they could build was 5 1/8". Even using that short a driveshaft we had to move the front axle assembly forward two inches so there was clearance between the fifth wheel plate and the oil pan. But it didn't stop there as we also had to move the six-way blade support arms forward, and we had to modify the transmission pan and the front driveshaft had to be lengthened. Finally, we had to lengthen the front steering tie rod.