My original 44-year old Goodyear R1 tires on original Kubota rims ('79 B7100) looked exactly like the pic you posted. I too purchased new 6-12 & 8-16 Carlisle Farm Specialist tires and the mounted 8-16 tires looked exactly like yours (waaaay too skinny).My rear R1 WHEELS are 16x8, and had 8-16's on them before.
I would like to follow your lead and install a set of the 9.5-16 tires, but I'm curious about your wheels. Did your actually measure them flange to flange, or are you saying they are 8" wheels because the old tires were 8-16?.
Supposedly 8" wheels started with the B7200 model.
Andreas