My plan exactly....

 

Going by parts listings the 5R and 3RZ have the same clutch "disc" so it would only need an adapter to fit the 3RZ to the stouts gearbox/transmission.
In Australia the first of 5 speed dyna trucks still had a 5R engine which was at first just a larger version of the 3R, the coaster bus had the same 5 speed version gearbox/transmission which was an upgrade of the stouts 4 speed and had 20 and 22R engines. After googling the 3RZ I stumbled upon one that was out of a coaster bus with the same style modernized 5 speed transmission leaving me thinking that relatively cheap complete used out of japan 3RZ with coaster/dyna style 5 speed column shift transmission being a near bolt in option apart from slight mounting changes as per Toyota

 

interested in finding out more on this!!

 

me too!

 

The 5R has many clutch disc or plate as they were fitted to different transmission such as the M40 and M50, although the R40 that the stouts had pre 1979 has a smaller input shaft which limits the things slightly, I do not doubt there is a clutch disc/plate with the overall diameter and input spline count and shaft size as here in Australia there are a couple of companies that make adapters to fit various engines to the stouts original R40 transmission including ford chev and Chrysler V8's on off the shelf replacement parts.