I have ran 2 Watson's and love them. Watson's came from one ton gmc and chevy trucks from the 60s.
I have ran both 5000 series behind a 78 c20 with 11.5 compression 454 mated to a sm465.
My newest one was behind a 400hp 383 with a sm465 before the c1500 went into the trees.
I now have a 75 cadilliac 500 mated to a sm465 sitting in the shed. The engine is stock but I could turn it into a 540 someday without drive train mods.
The Watson I have is rated for 5000 input torque. I have put almost 3500lbs into it on a regular basis dropping the clutch. I have twisted shafts, broke u joints, and gernaded yokes. The thing is a tough box that will handle abuse.
Mine is a 3 speed with 1.52 low, direct, and .72 overdrive. The parts can be bought through spicer if you dig around. But a entire box can be had for around $1000.
The thing is heavy at 160lbs off the top of my head and is a divorced unit meaning you may need a prop shaft and shorter drive shaft. The shifter requires you to have another stick in the truck.
But only if you want low.
If you just want direct and overdrive they are on one one of the two rods. This means you can put a silionoid on the direct/overdrive rod and shift it with a push of a button with no shifter or floor modification.
This let's you still split gears for the situation giving you 8 speeds. With no modification.
Having the 3 speeds really opens up opertunties. My driver right now is a 96 5.7 nv3500 with 3.42. I have had for a week.
If I install the aux behind the nv3500 I can put it in low and take off in 2nd for for 3.53, 2.13, 1.52, then shift into direct for 1/1 then staying in 4th shift into overdrive with the aux leaving the nv3500 in 4th with .72 ratio. This reduces strain on the nv3500 letting me tow. Putting the nv3500 in 5th would knock the rpm down to around 1800 with the 3.42 ratio. I can run virtually any ratio in the rear axle letting me run 4.56, 5.13 or higher with very few rpm difference but considerable torque multiplication.
With a th350 you can leave it in low then shift once your in in direct for better acceleration. 3.73 rear axle ratio becomes 5.67. Shift into direct and you have 3.73. Shift into overdrive for 2.69.
I have ran it behind automatics and splitting the gears does not work. You can do the silionoid idea and just use direct/overdrive with no shifter.
But putting the thing in low and matting the gas letting that thing get sideways before you hook while the needle just flies as the rpm increase is something that is hard to give up.
The thing is non electronic. You can add a dragster shift silionoid for cheap and have it on push button or rpm switch in 4th.
It is stout and overbuilt. I have been mean to mine and the worst damage was a broken tooth. It is as complicated to rebuild as a toploader transmission. The thing is divorced which means if you want to switch to a th400 or 200r4 or other you probably can if you add enough slip in the prop shaft with no modification.
This would let you drop in different motors/transmissions with many more options.
The downside is the weight and whine it makes. It is not electronic which means going from low to direct can be slow requiring you to bump the gas. From direct to high has netural in the middle. But you can yank that lever back while dropping the rpm and it falls right in after a bit of pratice.
It is not for everyone. But as you can see I love them.