restoring my Canadian 71 Dart 340 Swinger Special

Sounds like you're getting right on with the resto and great to see some 'before' pictures.
It looks like you've got your work cut out for you but it seems like you're the right person to tackle the job.

Regarding the tubs and rear end,that's a tough call but those darts do look awesome with the big tires on the back and since it's already done,I'd be inclined to clean up the work as best as it can and run it like that unless the tubs are in the way of the back seat.Also,since you're not dealing with the original engine,you can do hop it up somewhat as well and the extra traction might come in handy.
The cage will be a nuisance and if you have frame connectors your could go without.

What's the original color for the car? and was it a stick car or automatic?