I would say go for the 20V gearbox. The ratios will be right, and you can keep your old one for a spare.
I used to have an AE101 with a 4AGE 16V mated to a c52 from a 7A car. First gear was way too tall, and the revs were too low on the highway. To even think about accelerating on the highway it was necessary to downshift.
If you buy the box and the engine together, I am guessing it will probably come with a clutch and other misc spare parts too. good luck!