There's no such thing as "big block" or "small block" on Mopar 4-speeds, really. 361s, 383s, and 400s all came with 23-spline input shafts--just like a small-block--though the Hemi and 440 unit used an 18-spline input. You don't have one of those, or even the 23-spline close-ratio... and I know that because going by the aluminum main case and the flipped 3-4 shift lever (the front one), what you've got there is an extra-wide-ratio overdrive truck transmission (they never used long-tail ODs in any passenger car). However, it will fit and function in B/C/E-body models, as well as trucks and vans. It uses a 5.125" front-bearing retainer, which is small-block only, but it can be swapped out for the normal 4.80" unit used with muscle-era bellhousings, big or small-block... or, you can have the outer diameter machined down to 4.80" if you want.
Yes, it's an 833, and yes, it's plenty stout, but a lot of folks don't dig the gear spacing. It works best with a 3.23 or 3.55. The first gear ratio is 3.09:1 so anything deeper than that (like a 3.91) makes for a very-short first gear, and the .73:1 overdrive fourth makes anything taller than a 3.23 less fun in the top two gears (though a 2.94 gear would probably be livable).
I've had a couple of cars with that type of transmission, and I think all the ado about the gear-ratio spacing is overwrought. After all, the two middle gears are nearly identical to 2nd and 3rd gears in a 727 or 904, and you get the hard launch of the low first and the long legs of the overdrive. If you have a stock engine, or a nice, wide powerband, the trans actually works well... it's not so hot with a peaky HP monster, though. With gas being what it is right now, a trans like that makes sense if you really like to drive your 4-speed car.
I know one very hobby-prominent Mopar enthusiast that's had just such a transmission in his 1967 Hemi Coronet R/T for at least 20 years with no trouble whatsoever.
