Yep. G70x15s on a 15x6, biggest B tires till 1970. I think Hemis had F70x15s, certainly did in 69, I think in 68 too.The A-12's, I believe, but those we '69 1/2 only.
You may be onto something with the Imperial. The code book shows the option for ‘68. I really like the G70 size vs the F70I wonder what size the 9.15 X 15 tires on a 1968 Imperial would be in the newer ways of measuring.
Back in the 90s, the '62 Imperial that was my daily driver came with an L78-15 spare in the trunk, with 235/75-15s on the ground. After using the ancient spare one time without a failure, I changed it out for a 235/75-15.Most tires were 14 " back then unless the car had factory disc brakes.
When they were available depends on the brand/model.
Firestone Wide Oval was first starting in 67 and later Goodyear Polyglas in 69.
Closest modern size conversion of the 915-15 is a 235/75-15.
The replacement size for the 915 was the L78-15 back then.