Replaced 305 engine with 350, runs poorly at steady speeds

Fuel pressure, filter debris, fuel volume, carb not adjusted

I believe a 96-98 harness from any chevy or gmc would work. V8 to V8 not V6 to V8 the 305 and 350 would be the same harness

ok ran great before changes. right. you say unpluged computor to set timing.. yes.. do you mean the plug on the firewall passenger side that looks like a fuse holder kindof..?? (set timing connector ) (tan /blk wire)
did it run good during timing proceedure??? wont be MAP that is engine vacume has nothing to do with what you have replaced.
proceedure is to set timing . shut off engine , plug in.. then restart.
yes timing moves as per ECM specs. check that timing did no get out one tooth when you replaced distributor. retime . restart

CHEVROLET PICKUP 1996 1997 1998 1999 8-305 5.0L VIN 8th Digit M
GMC 1500 PICKUP 1996 1997 1998 1999 8-305 5.0L VIN 8th Digit M
CHEVROLET EXPRESS VAN 1996 1997 1998 1999 2000 2001 2002 5.0L VIN 8th Digit M 8-305
GMC SAVANA VAN 1996 1997 1998 1999 2000 2001 2002 5.0L VIN 8th Digit M 8-305

The Idle Air Control runs idle speed, it may be stuck or just bad. its located at the left rear corner of the throttle body, make sure its plugged in.

if this is a fuel injected engine youll most likely have to replace the idle speed control sensor this should solve that problem.youll have to get one for the intake that you put in the car(one for the year and engine size of the replacement parts)

If it does not do it in park or neutral, it could be a number of things. One being a sticking Torque Converter Clutch solenoid (TCC). It could be that it needs a simple tune-up. How often does this problem happen?

either your fuel filter by be plugged, could be injector shorting out could be throttle position sensor

I have run across this problem and discovered that it was the mass air flow sensor.

everything will bolt up the same the only difference between the two is the bore the 350 is the better engine of the two