Had this happen two times. The only thing I changed on the motors was the oil pump and pan. I flushed out the cooling system and put new freeze plugs in it. Both of them and replaced the oil pump put it back together put it down in a little red express and at idle cold had 40 to 50 lb pressure but as soon as it warmed up the lifters would all collapse. Thinking it was the engine we pulled it out when the first one did this and took a second engine that we knew had been okay out of a car change the oil pump, flushed out the cooling system put new freeze plugs in it, stuck it down in the little red express again exact same thing happened it would have 40 45 lb cold warms up all the lifters collapse the oil pressure drops to zero. Only thing we can figure is the oil pumps are bad never got either motor to carry oil pressure. P0'ed so bad the truck has been sitting in the barn for 18 years now. Both motors only change was the oil pump and pan. Someday I'll replace the motor agin.[/QUOTE]
First of all why would collapsing lifters cause a massive pressure loss?
The OP has 1 of 2 problems--It has wiped out main bearings or excessive lifter bore clearance, because of the clicking on #7 tells me you have nice and sloppy oval lifter bores which means the rest of them are likely worn out too. This is your pressure loss when warm. A normal healthy SBM will have 60-70+ psi cold idle not 40-45 psi. My Cheepy Three-Sixty build engine has 65 psi cold idle and 25-30 psi hot idle and it doesn't have the best lifter bores. J.Rob