Wrong Tires On My Truck?

[1969_CM2590D]

I've had a bit of a surprise tonight. Let me go in steps first:

1. Truck is a 1969 GMC C2500

2. SPID says it came with 3R67BA 7.50x16x6 front
and 3R67AL 7.50x16x6 rear. Transposing from the 1969 Chevy Truck Fact Book I have stored in the PC here, option code "R67" is for a 7.50x16 6PR tube tire with a 6" rim width.

3. Measuring from bead seat to bead seat by the moonlight and flashlight right now, I get 16". The rims look to be the original 8-lug argent face with a fair amount of surface rust

4. Truck now has 8.00x16.5 front and 9.50x16.5 rear bias belted tubeless tries. The spare is an old dry rotted Uniroyal 8.00x16.5. These tires were on it when I got the truck. Tires hold air fine

So do I have the wrong tires on there? Can you mount 16.5" on a 16" rim and have have it hold pressure? Am I measuring the bead seat wrong?

Thanks