pulled codes, please help

[84LX89GT]

god i hate EVP sensors, had constant problems with EVP signal in the 89 GT i had only to find out there was high resistance in the signal wire from the connector to the pcm somewhere hidden far away.
-the EVP sensor, or E.G.R. valve position sensor is a sensor with a plunger than mounts to the egr valve and sends varying voltage depending on the position of the egr valve. Chances are you have a bad EVP sensor or faulty wiring make sure there's no corrosion in the connector.

67 neutral circuit failure - don't worry about it especially with an engine swap unless you feel like doing more wiring if it's not already installed.

29 see what happens with new gear, you may have a bad vss sensor or wiring

56 either the mass air flow sensor is faulty or you have a wiring problem. It sounds like either the MAF sensor is bad or there's a wire shorted to power or miswired.

[Hozer 88GTConv]

I'd get an EGR simulator plug from ebay. Some guy in Colorado sells them. All it is is a male plug that fits into the egr plunger harness and sends a 2.2vdc signal. That will eliminate the egr code.

The MAF code means you need to check the signal wire from the MAF itself AFTER you clean the element with evaporating electronic parts cleaner.
If a cleaning doesn't work, then check the signal voltage on the appropriate wire (Tan/Light Blue wire 18 gauge is the signal return, dark blue/orange is the signal wire)
Or go to a junkyard and get a MAF sensor from any FORD from the right period, I paid $5 for a couple of 'em. (at the parts store new, they were $200!!)

A bad VSS will hurt your drivability off of the gas, as the car returns to idle off the gas it may hang up or hunt/surge...take the sensor off and check voltages or swap (they are cheap!)

good luck...