Best just to remove the manifold and tackle the sensor issue on the bench.
Or you can force it insitu and hope that the thread in the manifold says intact.
The cheapest and easiest fix would be the manual cloth seat option.
Unfortunately, I do not have any stock at the moment, however, you should be able to pick up a pair for £50.00
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.