I'm going to make a suggestion as something that you can do easily just to road test and see if you happen to have a problem in this area.
I recommend that you remove the vacuum hose from the EGR valve and plug it, and road test the truck.
If you surge goes away, then you have isolated the area that needs attention.
Let me know if this helps to reduce the surge with the EGR valve not functioning.
The smog pump should not really cause this type of a drivability problem.
A partially restricted exhaust system or partially plugged catalytic converter could cause a surge usually noticable under a load.
If you have a restricted exhaust or a fuel filter that is plugged, you will almost always notice this when you come to a uphill grade, and the vehicle will actually slow down as you begin to attempt to give it more accelerator pedal.
Without being there to drive the vehicle myself, I'm wondering if it is absolutely a engine related problem. We did have some problems with the TCC (torque converter clutchs).
This can feel like a engine problem, if the TCC either hunts in and out, or stays engaged under load conditions. The TCC may also chatter and feel like an engine surge.
Just tossing out some possibilities.
I'm going to do a bit more thinking on this one...
If you are thinking Smog Pump, you can easily pinch off the hoses that run to the check valves and to the catalytic converter and drive it again.
I really thought by disconnecting the EGR so it wouldn't open, would take care of your surge.
Does the engine Idle nice and smooth or does it Idle rough?
Here's what I would do next.
First lift the EGR valve pintle while the engine is at Idle. Simply reach your finger underneath and pull the diaphram upward, and verify that the valve is not stuck in the opened position. You should notice a very rough running engine while lifting the valve, that will smooth out again when the valve is released again.
The next step if not already done, would be to thoroughly check for any vacuum leaks that would be causing a slight misfire at Idle. Listen for what sounds like a hissing air type of leak, and even a can of carb cleaner spray can be used to spray areas of possible air leaks.
You can next Pull one plug wire at a time, and listen to the engine RPM change. If you pull a plug wire, and the RPM does not change as on the other seven, then you have isolated a weak cylinder.
And the last step is do a good compression test, to verify that each cylinder is functioning.
If you have a valve that is only slightly burned, and there is some compression in one cylinder, but the rest have more, this may seem OK, until you get up around the 30 MPH range, where that low cylinder compression may not be allowing for combustion in that cylinder.
Seems like that year big block should have about 150 lbs of compression, and if 7 are at 150 to 160 and you have one at 110, you may have isolated the problem.
I hope this information is helpful.
By The Way,
If when you lift the EGR pintle, if there is not a significant RPM change, then the EGR valve is allowing Exhaust gassed to pass through at all times or the passages are plugged. Either way, this should then be on your list of things to fix.
Although a plugged passage should not cause the surge, a stuck open valve would.
Since you had a fire, you should look closely at a possible intermittant connection in the related wiring. An intermittant Throttle Position Sensor can really be difficult to isolate. Has the TPS been replaced since the fire?.
Try the wiggle test on all of the related wiring as well.
Ya know, GM isn't as concerned with the compression pressure readings as how even the cylinder to cylinder compression readings compare with each other. Keep that in mind.
Thank You for the "Accept".
I sincerely XXXXX XXXXX could be more help, however there are some problems we encounter that are extremely difficult to isolate via long distance. I always try my best.