MSD 5520 any good?

Basically all you will be doing is using an HEI output capable coil with the MSD box.
It'll work fine as long as you stay within the ohm rating the MSD box wants for a coil.
You would do yourself a big favor by spending that money on a performance oriented distributor instead if you don't already have one.

Both the MSD and your HEI need a coil, so what you would be doing is replacing the HEI module with $140 MSD box.
(Just so you realize that)

I can help you with the distributor if you need it.
Just message me.