Is this the original 2.9 with the original computer? Just wondering as you mention you replaced the MAF as SPEED DENSITY was utilized on the 2.9 until the 1990 model year in Calaifornia delivered units. Have you checked working fuel pressure? Have you verified that both fuel pumps are working? There is a LOWpressure pump in the tank and a HIGHpressure pump on the frame rail. The LOW pressure pump feeds the fuel filter canister and the HIGH pressure feeds the engine. A failing or failed LOW pressure pump will cause major drivability problems as the HIGH pressure pump is not designed to suck fuel from the tank, only to pump what is fed to it. Did you replace the cannister fuel filter or in-line fuel filter, or both? A shorted O2 sensor will also cause excessive fuel to be delivered without setting a code. Worn rings, valves, valve seals, compression test?
Simple diagnosis for a plugged cat: A common vacuum gauge will help, attached to the intake manifold. Manifold vacuum will normally reduce when you first tip in on the throttle. On a steady throttle position, the vacuum will rise as the engine rpms rise and the load is reduced. With the cat plugged, the vacuum will stay low. No manifold vacuum means that the engine can't draw any more air/fuel charge.
NORMAL MOTOR: Steady reading of 17-21 when motor idling. NORMAL MOTOR: When throttle is opened and closed rapidly needle falls to 2 and swings back to 24 or 25, falling back to normal idle reading Indicates rings and valves ok
Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too low) - ISC
ALL - (O, M) EGR did not open/ respond during test or if memory code, did not open intermittently - EVP EVR PFE
Park/Neutral circuit fault - PNP
Transmission Manual Lever Position (MLP) sensor circuit - Transmissions
(M) Intermittent Park Neutral Position (PNP) sensor fault - PNP