can u wire up the gm hei and still run my msd 6a or is that overkill. if its too much which would be the better choice

You use one or the other, not both. Unless you have a "race" car, you don'r need to spen the money for a MSD 6.
The HEI is more bang for the buck, and will supply all you need on a stock or modified street car. I have the HEI on my "low buck" race car.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Although I am a supporter of the HEI swap, I'd say that if you already have the MSD and it works okay, use that.
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Ford Futura picture

You can have the HEI trigger the MSD if you really want. The only benefit to that is if the MSD fails you can switch back to the HEI easier. Personally I'd set up the MSD to trigger directly from the distributor and put the HEI parts in the glove or tool box.

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Joshua