I would be more concerned about octane rating than lead content.
Typically, lead-free gasoline works fine and valve-seat wear is not excessive.
Granted, the pre-smog era engines were designed to run on leaded gasoline, and yes, flame-hardened valve-seat cylinder heads were the early factory "fix" when lead was banned ('452 heads,) but most people who run the earlier non-hardened valve-seats, wear is not excessive.
There are exceptions, but it's not as if you drive for 5,000 miles and need a valve-job.
Quote:
From 1974 to the end of big-block production in 1978, several different castings were used--the 902 in 1974, the 975 in 1975, and the better-known 452 from 1976 to 1978.
All these heads were minor variations on the 346 casting from 1971, with differences associated with durability details such as hardened valve seats, improved crack resistance, and an enlarged guide boss in the valve seal area.
True, to be 100% safe, you would need to have hardened valve seats installed in the earlier style heads, or you could use artificial lead additive or low-lead aviation gasoline, but I sincerely doubt you'll see any significant wear issues using unleaded gasoline.
Here is some information on Factory Mopar Cylinder Heads:
Mopar Muscler Magazine Cylinder Heads Information
