I have installed one of GRONK's Motorcraft 2100 kits on two AMC/Jeep 4.2s in the past, and enjoyed the results. Well worth the amount he asks for it.
ECM Test "test" is required for any non-factory-computer-controlled carb, like GRONK's 2100, or a swap to an earlier non-computer Carter BBD.
No need to hollow out the cat unless it is known to be bad, as in rattling/chunks moving around inside it, or you've seen it glowing from trying to handle excess fuel.
Might give the intake manifold bolts a torque check/carb clean spray test while running, they do have a tendency to back out and the resulting vacuum leak can make any kind of carb problem impossible to diagnose/fix.