Arch, (Can I call you Arch?)
First, welcome to the forum!
In my opinion cab conversions run the gamut from beautiful custom creations, with limo-like interiors, to scabbed together Bubba and Billy Bob hack jobs. The machine my snowcat buddy and I re-cabbed was converted from a 1642 to a 1643. We tried to follow the basic Tucker design, though with improvements to enhance comfort and functionality. The basic fabrication is not particularly difficult, but it is very time intensive to get everything right without "cheaping out" or taking shortcuts. I'd estimate we had at least 200 hours in the cab conversion alone.
My honest opinion is you'd probably be ahead by trading your 1642 in on a nice 1644. Right now, the only used 1644 I'm aware of is at the Tucker factory. It's a 1980 model, 318/5-speed that was owned by an electric utility in Montrose, CO. That said, your 1984 model has some design improvements that are, in my opinion, more desirable than what came on the 1980 versions. Specifically your carriers have damper wheels in lieu of the hyfax and bracket system, and you have a tilting hood instead of the butterfly style used in 1980. Perhaps you could work a deal whereby those parts would be swapped? Either way, I'd highly recommend you call Jeff Godard at Tucker to see what that would cost.
That number gives you a baseline to compare against the cost of having your machine converted. Based on personal experience, there's quite a bit to it. You remove the roof and bed and basically cut off the cab behind the front seats. You'll have to build the cab structure and a new roof, including some type of roll bar for protection, as well as supports for the floor, and a new floor for additional cab area. You'll have to shorten the bed, and sideboards as well. As the owner of a 1644, and as a guy who's 6'4", I'd suggest making the cab 6" longer than stock so rear passengers have some legroom. That means either shortening the bed that much more, or stretching the frame. That's really not that big of a deal (we've done it), however it also entails getting the rear driveshaft re-tubed and lengthening the rear steering arm, in addition to some minor fuel line and wiring modifications.
But you're not done. After the cab and bed mods are finished you'll need to get it painted, and it's likely you'll decide to get the whole cat painted so it all looks the same. I seriously doubt you'll be able to match either the seats or the upholstery, so you may end up replacing the front seats as well (maybe new seat mounts in front, too) and get it completely reupholstered. I think you could easily spend $10K by the time you're done, assuming you do a first class job.
