Ah, man… What a drag! Good luck, hopefully it turns out to be something simple. You did the smart thing by sliding back the converter and isolating the accessories before digging into the engine. I had a water pump fail on a 454HO that sounded just like a rod knock. Scared the hell out of me! And to make it worse Coloradobill and I were on top of Mt. Antero when it happened. We had to make a 3 hour ride back to Salida to get a water pump then change it right next to a stream bed. Ah, yeah….bunny huggers should have ******** themselves seeing all of the coolant leak right into the stream when we popped that water pump off! (just kidding, we caught all of the coolant in the drain pans we bought at the water pump store and proudly never spilt a drop :sombrero

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It seems engines aren’t cheap to rebuild these days either. Hell, here in Pueblo there isn’t even a machine shop to use if one wanted to build an engine. Everything must go to Colorado Springs or Denver for machine work. There is one guy in town that does heads but nobody is doing machining.
Wholy cow, why do you need to go through the hassles of pulling the radiator support off? I’ve had many of engines in and out of my 6” lifted K10 over past 17 years for various reasons (clutches, oil leak repairs, etc) and always pulled it right over the radiator support not to mention countless big trucks at the my dad’s shop in the old days. Just place jack stands under the frame, remove the front tires and lay her low. Your radiator support is even shorter on an 81-91 than the old flat nose 73-80. My cherry picker is nothing special either
Getting a 454HO back in 2003. Frame on jackstands
8.1L infusion in 2008. Frame on jackstands again