Joe, it's been done.

If you want to do that "on the cheap" you could make a factory distributor belt drive and drive it off the crank at half speed.

For carbs, someone was selling an intake to take Webbers (or similar side drafts).

I'm sure some resourceful soul could make a multi pot downdraft manifold as a one off, but I don't know how well it would work.

As I recall efi-diy locked his exhaust cam with parts from an intake cam drive, I also seem to recall it's somewhat more developed now. ;\)
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My, what a steep learning curve. Erik II#5155