My '91 wound up with water in the gas tank. Before I caught the problem,it had ruined the fuel pump and sender. I have a spare fuel pick up unit and pump for my '85 and would like to use it to replace the damaged unit. Other than the weather pack wiring connector is there any other difference between the two that I should be aware of?

Any input would be greatly appreciated.  

Both cars had multi-port high presure injection, so the pump should be fine, As far as I know the tank never changed int he C4 so the sending unit should be fine also.

The weather pack fitting difference is something to consider. I don't know hwo the '85 unit plugged into the car's wiring harness, but it's tough to beat a weather pack fitting for reliability and keeping moisture out.

Have you thought about trying to get a '91 specific unit of eBay?  

All depends on the the fuel pressure of the 91 and the 85. Iknow that the 8th digit of the vin will tell you your engine code because diferent engine codes have diff pressures. MY 90 corvette has 8 code eng @44lbs the J code engine same year @35lbs fuel pressure so i would find out what size pump you need .