Knock after a simple carb swap - Thoughts?

[Android04]

Yes sir you read the title right... I've worked on TONS of SBC and BBC in my days and never have I had this issue. It may just be the luck of the draw in that exact moment. As I say "if I didn't have bad luck I wouldn't have any luck at all"...

Vehicle: 1985 Silverado Dually (C30)
Engine: 400 SBC
Trans: TH400

So I've been working through some leaks the truck has had and got it running and switching gears. New intake manifold gasket, new valve cover gaskets, new thermostat, and all new fluids. Then noticed the governor was chewed up so I put in a new one in the trans, with a new trans filter, gasket, and fluid. Yesterday I decided to drive it a short distance and it did great. I knew the carb was having some jet issues.

So I swapped out the carb for once I had laying around that was perfect on a previous project that is now on Holley. Swapped it on my truck, started it, idle was a little high but seemed like the throttle cable was too tight. Adjusted it and BOOM... A super loud knocking sound. Something it has NEVER done.

I first thought it was a fly wheel bolt maybe, or something smacking. No luck. So I pulled all the plug wires off one by one and the noise or pitch never changed. Still has great oil pressure. But if I start it and turn it off it sounds like a rocker arm is still moving for a solid 30seconds, and sounds like it back by #8.

Although I'm afriad its in the bottom of the motor I wanted to see if anyone had any suggestions, thoughts... etc.