As you say not all 350s have the electrics.
Full electric seat wiring looms and harnesses are in stock if you decide to upgrade.
Another option would be to fit a pair of manually operated cloth OEM seats which come as standard on some of the cars.
Gary,
If your issue is fuel related then I would be looking at the fuel syphoning system.
If this is not functioning properly then a quarter tank on a sharp/fast right-hander will cause severe lean out.
Keep your tank at least half full and see if the issue returns.
Not part of the service, but best to change if the fluid is dark or has boiled.
DO NOT let the reservoir run empty.
Remove old fluid from the reservoir with a syringe, top up and bleed.
Repeat until all the old fluid is out.