On a 66 442 how difficult and expensive would it be to convert the distributor from points to HEI?

 

Simple approach would be to install a under the cap system like Pertronix, Crane, etc. Looks stock and runs better. All you'll need are simple hand tools and an hour or two.

 

How expensive would one of those units be?

 

Between $90-120 depending. I've had experience with the Pertronix unit and they are excellent and the performance increase was noticeable. Just remember they need 12v to work properly so you can't use the resistor wire that goes to the distributor now.