Anything except a stock type 3- or 4-speed is going to mean a change of rear axle. If you don't mind changing the rear I think the Japanese car reference suggests to me she'd be happiest with the T-5.

Actually I guess there is a kit to convert a Saginaw 4-speed to closed driveline so that would be an option with the stock rear too. An advantage is that a vintage look shifter is available for the Saginaw too.

As to whether you go with a 235 or a 250 that choice is really an independent issue, since you can run any of these transmissions with either motor.

The 235 of course is easier to install than the 250, since the 235 is the same mounting as the stock motor, but the 250 can be done without a lot of trouble.

The motor decision can drive from the look you want, or perhaps from what you find most readily available at your price or in the condition you want.

A few years ago a friend of mine was selling a 250 and Powerglide out of a Camaro. It's long gone now, but at the time I sure considered that package for one of my projects. Would have made a very nice driver.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...