I'm not sure what Jim means by variable venturi carb. I had an 85 so I know the engine reasonably well. It had a Holley which worked just fine. It also used the Duraspark II which also will serve you quite well. You made a GREAT choice for an engine swap.

If it were a point ignition, putting +12V to the coil would indeed be what you would need to do, but you will be connecting the Duraspark box to the distributor and possibly coil, and then putting your switched +12V to the Duraspark connector somewhere.

I expect that a little googling will get you a wiring diagram for the Duraspark box, and the Duraspark II box is readily available at any parts store. Simply buy one for an '85 Mustang GT.

BTW, there was a bit of a marketing deception involving the roller cam in the '85 cars. Only the ones produced beginning in mid year, February I think, had the roller cam, so don't take any chances. Use an oil with ZDDP additive such as Delo, Delvac or Rotella as if it had a flat tappet cam, if it does have rollers it will still serve well.

Hope this helps and good luck with the swap.