At a guess, your fuel sender is stuffed - fairly common problem with these models. May pay to do a continuity check on the wiring between the gauge and the dash to make sure.
As for the trip computer, does it have any display at all? If it is not working at all it is probably a power fault. Get hold of a workshop manual, which will tell you which pin(s) in the connector feed the power, get a multimeter or test light and check for 12 V.
The 351C will fit in the ZL using standard engine mounts, but I understand it will need an engineer's certificate, as these cars were never fitted with a V8. There are problems with heater hose clearance, and you also may need some reinforcing of the crossmember (someone else may be able to confirm that last point).
Power steer and air can still be used, but you will probably need new brackets to mount them with the V8. Different hoses may also be required.
The 9" is probably designed for a leaf spring rear end, and will not fit under the Watts linkage coil spring rear of the ZL without modification. Any decent diff works could do this for you, but it would cost.