That's a nice looking jimmy you have there. Knocks can be anything from a valve adjusted way to wide to piston slap to bearings. I'm not quite sure how your friend decided it had a bent rod, but doesn't make sense to me. I've been working on GMCs for over 50 years and I don't think I ever came a across a bent rod that caused a knock. I am to far from Tulsa to know any mechanics, but there has to be some old timers around still working. You might pull the valve cover and try sticking something between the rocker and the valve one at a time while its running and see if it makes any difference. The intakes are supposed to be .012 hot and the exhaust .020 hot.