Torsion Bar Replacement

Here's my opinion.
It's been my experiace there was no difference in size between what generally was in 6 cars and 318 cars. Check out the size before you do anything.
Did you say you have a sway bar front end?
If that is the case, I would recommend trying it first. I'm very please with my poly front ends in non-ac, manual brake and steering cars.
(Including the lower control arm poly bushing which tend to make some people nervous).
Changing the torsion bars at a later time would only involve an alignment at most since the you might not get the ride height back perfect and that would change things..