budget street motor 360 or 383 for 71 duster

In my humble opinion I would use the 360 and here is why:

You already have it

Some degree of weight advantage depending on aluminum heads etc.

More room in engine compartment

No additional parts needed, ie mounts, driveline stuff

Horsepower goals could be easily met using good heads, intake, cam, and good pistons. At those power levels your bottom end would be up to the task with a refresh, provided it is in decent condition.

Your Duster is a lighter car than a Chevelle, a nice 360 streetable combo should easily embarrass the 383.

The feeling of satisfaction of winning with a smaller engine will be great!!!