Best tire pressure for 69 dart with P245 60R14

Using BFG's T/A 205 70r14 as typical example, recommended rim width is 5-7". The optional 14x5.5 JJ rim should not be lifting tire tread off the road. Are you sure it doesn't have the standard 14 x 4.5 JJ rims?
Reducing pressure also reduces its load carrying capacity.
Going to 215 70r 14, the minimum width is 6". I've run that tire on a 6" rim with no unusual wear. I'm running a Firestone in that size right now (no longer available).

Ironically, the BFG Radial T/A is a relatively cheap tire here. Go figure.

If by Magnesium you mean real magnesium alloy, they need lots of care and careful attention. Those are really race only rims for situations where lightest weight was required.

If you mean cast aluminum, most are relatively strong, available new and used in the small bolt pattern at reasonable price. Some care is needed during tire mounting. Sure they can be cracked, but not a major concern. My street tires are on an old set of no-name cast aluminum slots. That's what is in my avatar. My competition tires are on new 15x7 ARE 'Ansen slots' Decent strength to weight. Not thrilled with how they look, but that was secondary.