Will my 8-3/4 hold up behind a 4 speed?

I have a 68 Barracuda 340, 4 speed so allow me to chime in.
In your situation I feel your 8 3/4 will be more than fine.

For me..... When you get to the point where you start launching at very high RPMs on slicks or running much faster it is time to go to a 60.

I had to change, yes HAD to change when 8,000rpm launches simply destroyed several 8 3/4s.