Should I do a Cat delete on '75 Duster

I agree with 72blunblu. There is a good chance your Cat is plugging up. Given you're exempt, axe it.
BUT, Chrysler Corp was notorious for lazy timing and very lean A/F ratios. (Lean Burn)
As others stated, get some timing in it AND jet it up. It will be overly lean with the Cat removed.
The HP rating reduction was mainly due to compression being lowered to be friendly with unleaded gas.
The gross/SAE hp ratings took place in 71 and 72