The Mallory unit, although a Hi Energy Ignition type distributor, is not the same as a GM HEI distributor, which has the coil in the cap and is considerably larger in diameter than either the old points or Mallory Unilite distributor. The Unilite uses an external coil (similar to the stock unit) although a 40KV or better coil is recommended. If you're talking about the Unilite (red cap type) then you can gap the plugs around .040" to .045" versus .035" for the points type distributor. I have a Mallory Unilite distributor (small red cap type) that I bought back in the 80's when they 1st came out. At the time I was drag racing a Camaro so I didn't get a vaccuum advance with it. You can dial in mechanical advance with the stepped key supplied with the unit. It's been on top of several engines and has a bit over a hundred thousand (yes 100K miles +) miles on it. The only part I've had to replace (besides the cap and rotor) was the electronics unit when it burnt out at around 60,000 + miles. I always gap the plugs at .045" and have never had a problem. You can use any plug wire, although a solid copper core wire will interfer with radio reception.
The Pertronix unit fits under the stock cap on the stock distributor, replacing the points. I've installed this on several cars for customers that wanted a "stock" appearing ignition. It works really well and is not very expensive.