The top circuit is simple; a dedicated +12 through a breaker, to SPDT on-off-on switch to the motor/pump. (Although the breaker could be shared with power windows or power seat.)

You said you had fuses for the top originally? From the factory, there should have been a 30A self resetting circuit breaker on engine side of the firewall. Had a previous owner done some changes to the original circuits?

Did the '68 come with the power top from the factory? If a manual to power top conversion take place at some point, that might be a source of the mechanical resistance. The frames are different.

The power top switches are horrible. I've gone through several over the years on mine. The high draw, stresses the contacts, burning and pitting them. There is a discussion here about an owner adding a relay circuit to eliminate the high current on the switch. https://firstgenfirebird.org/forums/ubbthreads.php/ubb/showflat/Number/243940/Searchpage/3/Main/27731/Words/%22top+switch%22/Search/true/re-power-top-wiring.html#Post243940

I checked several of my documents and motor/pump vendors. Odd that nowhere can I find the current specs on the part. 12v seems to be stamped on it but no AMPS.