As far as posi: https://www.speedwaymotors.com/the-toolbox/what-is-positraction-and-do-you-need-it/53704

Hot rodders generally consider posi a desireable thing. Is it really necessary? Up to you.

Is your 4" drop frame to ground? Frame to body?

You might consider looking through some of Coilover's old posts to see if he has a recommended tire size. His shop does a lot of mustang 2 conversions.

From the Rocky Mountains?
Check in with the RM Bolters!
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