'66 rear bath toilet

[FreshAir]

I am about to replace the original toilet into my '66 Tradewind rear bath AS. I have replaced the floor and black tank. Before I had removed the toilet I noticed that the top rim of the bowl did not meet the fiberglass bench it sits under by about 3/4 of an inch. The bench would flex down to the toilet when a person sat on it. I thought it may have been from a sagging floor. The floor has been now replaced but the toilet is still below the bench. I tried shimming the toilet up but after 1/2" the floor screw in-flange (to black tank) loses it's connection. I was thinking maybe there was originally a rubber collar or bumper that was supposed to fill that gap from the top of the toilet to the underside of the bench. Anyone know if that is true or any suggestions to correct the problem I have?
Thanks,
Neil.

[dibAL]

My '66 had a shim between the top of the bowl and the bottom of the bench. The interior was bridged with a plastic trim ring. The somewhat delaminated shim consisted of two pieces of 1/2 plywood cut to the bowl dimensions. It was anchored by the seat bolts extending through the bench. I'm in the process of replacing toilet; should this be necessary again, I will probably fabricate something out of plastic (though the plywood did last 40 years). Hope this helps, mike