1997 Chev Vortec - Replaced FI spider, now wont stay running more than 5-10 seconds...

I have a 1997 Suburban 1500 4x4 w/Vortec 350.

Have had continual error codes for O2 sensors, Multiple Random Misfires, Mixture Too Lean, some others.

Was told probably had injector blocked, so did injector cleaners few times.

Now got to running very poorly - not all cylinders seemed to me.

So talked to few mechanics and told me time to replace the fuel spider to the upgrade Multiport FI. I bought the OEM Delphi unit.

FYI - Previously had replaced Fuel Pressure Regulator, and that helped instant start.

Opened up the upper manifold, and it was all carboned up inside cover - totally cleaned it.

Top of the lower manifold had wet black sludge all over it.
Cleaned it to as good as could be without making it a career,with paper towels soaked with carb and brake cleaner to breakup the cooked and wet sludge.

Got it pretty clean, including intake ports as far as I could reach in.

Took strong shop vac to all areas cleaned and into each intake port to make sure no pieces of paper towel residue left - cleaned as best I could.

Wire brushed FI port ridges in manifold, to clean all 8 of those.

Installed the fuel spider - the ends to the outer FI ports, the next rolled inner FI lines over the outer FI lines to the inner ports, per instructions.

Put everything back together, and was looking for motor to work like new, but it start sounding strong, but runs for 10-20 seconds and then dies.

I can get it to run a little longer by giving it more throttle, and then just dies, and did get a few back fires.

Looked over all connections and base installation of the upper manifold appears to be flat and sealed.

BTW - When I put in the FRP previously, I used a little HI Temp Copper RTV on the upper manifold base gasket, but did not do that this time...

Should I have, as I did before?

Parts store said no with rubber gasket...

Worked for me before with the FRP replacement fix about six months earlier.

I gave up last night, as it was late and thought I would see what you guys think???

I did this repair without pulling the lower manifold, as the internet videos I watched showed it to go just fine that way...

Ideas for what I need to do to straighten this out.

Appreciate some corrective suggestions for next steps for me to resolve.

Right now, thinking I have to just go back in verify FI cylinder correct routing, and put it back together again.

Thinking this I will add some RTV to the upper manifold base gasket.

Maybe its a base air/vacuum leak?

Thoughts?

Frustrated in Minnesota...

Thanks
Lfryklu