Anyone have Holly EFI on 360 Magnum? Issues?

For starters, you'll need an EFI electric fuel pump; and depending which pump/regulator setup you go with, possibly a 3/8 fuel line and a return. If you're going with the Holley in-tank pump/sender unit for instance, it's already self-regulating and has an internal return. Either way you need to run power back to the tank.
I believe the Sniper still requires the Holley/Mopar throttle/kickdown linkage adapter.
Depending on what intake manifold you have and whether it has an extra temp. sender port of the correct size, you may need to get a thermostat housing adapter to run their sender.
And then there's the distributor; if you want the Sniper to control your advance, you'll have to either lock out your current distributor or get a different compatible distributor.
Lots of threads on all these topics, search and enjoy.