I've done the NV4500 swap in a 59 GMC 1/2 ton, and I have everything ready to do a swap in my 1940 GMC 3/4 ton.
As mentioned above the NV4500 is a bit more expensive to install, but it does offer several advantages. You lose a little bit of gearing in 1st gear compared to the sm420. First gear in the sm420 is 7.05:1, while the NV4500 is 5.61:1 which is still a bit lower than any t-5. The NV4500 is rated to take more torque. The NV4500 is a truck transmission.
T-5s are cheaper, more plentiful, and bolt up eazier than a NV4500. On a 235 I would probably go with a T-5. I chose the Nv4500 in both of my projects because I was looking for more transmission than I needed. In the 1940 I will have a heated up 302 GMC 6. That one should put out some good hp and torque numbers.
On the 59 I put the NV4500 behind the original V8. I just thought the T-5 would have been too light behind the V8
