New to this branch of the forum, been in the 61-66 forum for years.
I bought an 82 with a 351w/C6 combo.
A little history:
Truck was owned by older fellow who took good care and kept all records since new.
He died and son drove it for about 2 years.
Carb started giving him fits so he parked it. For 7 years!
I brought it home, oiled the cylinders, and rocked the engine a few degrees
and it rolled over. Never was actually stuck, I was just making sure.
I rebuilt carb, changed oil, new plugs, cap, rotor, etc.
Truck started and ran fine until it started getting hot. I replaced the t-stat,
and had radiator flushed, then finally , a new water pump. Truck ran fine for
about a month. I then drove it out of town. Here's where it gets weird.
At almost 60 mph cruise, the engine sounds like a rod knocking, really loud.
I apply a little throttle, the noise goes away until I start to let off.
SO, under load and on decel, no noise, but only at cruise between 60 and 65 or so.
Definately the engine, because it doesn't make any noise in neutral at those speeds.


I just did a compression check:
#1=105
#2=115
#3=110
#4=95
#5=110
#6=120
#7=110
#8=105
Are these in the right range or should they be 150 and up?
Also, when the engine is shut off, I hear a lot of ticking for a few seconds
and then it stops. Kinda like when it cooling, but it does it even if it's cold or
warm. I'm thinging the lifters are leaking down rapidly when the oil pressure
ceases. Any thoughts?

 

Those compression numbers, while low, are fairly average for a 30 yo engine.

All cylinders in a healthy engine will be within 10% of each other; yours is at around 20%.

But that's not gonna cause knocking.

Hook up an aftermarket oil pressure gauge and look at the oil pressure - cold idle, hot idle, cold 2,000 RPM, hot 2,000 RPM.

 

It sounds like piston slap. When you lay into the throttle it goes away because the vac drops,when you back off and cruise its making vac and pulling the piston up. My old 390 has this because when i was 16 i didnt have money to replace all 8 pistons being a ring stuck and cracked one, motor was about .003 over standard due to wear. Put a new standard size piston it there and it did exactly as you describe and its going in 20,000 miles. Turn the radio up until you can rebuild it proper.