Should I do a Cat delete on '75 Duster

The early cats were restrictive. In my view the big change in that aspect came with the Ford "5.0"
Now we can readily buy low restrictive cats, regular and '3-way', etc.
That said, for a 225 turning 2500 rpm, cruising, not much restriction.
IMO the biggest killer of torque and throttle response in a '75 (vs a '67) is the timing and timing delays.
Do some reading up on OSAC, and look at bypassing that. Delays when coasting a braking are no big deal, but not having the right timing for power as soon as the throttle is opened is a huge deal.
Chrysler acknowledged there might be complaints about drivability when system was introduced in 1973.
see here
Master Technician Service Conference - Chrysler's Training for Mechanics