I,wouldn't worry so much about being off on the housing; the H190 is 3" wider than stick (1.5" per side) and it first bare fits. Note I did have to flatten the fender lip to get the room I needed. So basically if you were off by 1/4-1/2" it's not,going to be a disaster.

In actuality I'd figure out the maxim track you can get away with, the wider the track the better the car handles.

If that is the case I feel like I should just go with a Volvo 242 rear axle then. It is a dana 30 which would be plenty to handle the power and there are a number of LSD/locker differential options to put in it. The width is 53". So roughly .75" a side.

Great idea. The Dana 30 uses a 7.2" diameter ring gear (~183mm), so plenty strong. With disc brakes too

The Volvo 240 Track is 1360mm (53.5"), which is the width at the tires center-to-center. The axle housing itself is wider. After adding the stock +25 wheel offset, rear axle width I think would be 1360 + 25 + 25 = 1410mm (55.5")?

Datsun 1200 rear axle assembly is 1305 mm (51.38 in) drum-to-drum