I looked up (O'Reilly auto) the pumps for my 81 and your 85 both with 300 six.
They only list 2 pumps for my truck and both show 25 GPH
I looked at yours and they list 2 pumps 2 at 25 GPH and the 18 GPH
The 2- 25 GPH pumps are the same pumps listed for my 81.
So with that said go for the 25 GPH pump
Dave ----