I asked around about the Mallory rev limiters, the limiter electronically grounded the tach wire while reading the rpm. So for you overdrive, it would be the same hookup as GM did, ground out the negative side of the distributor ( HEI negative side is the tach signal ).
" On a HEI the tach wire terminal is the same as the Neg post on a conventional coil.
On the Mallory Tach the Rev Limiter simply grounds the green signal wire to the black wire.
Some drag bikes use a micro switch to temporary kill ignition while clutchless shifting and in road racing you can get this feature for a dog-box [Flat shifting] where a micro switch is connected to a load cell on the shift lever."
It will be a much easier job using small 18 to 20 gauge wire with no amps involved, you can even use the original GM relay. The other plus is it will be easy to disengage if you ever want to remove it, the main power feed will still be one piece with no relay to worry about.
As always with HEI, throw a spare module in the tool kit. I've carried the same one in four different cars for 40+ years and never needed it, but I know I have it if I do !
