[MagicRat]

I will assume you have good oil pressure and that you have blown out all the pushrods with compressed air and the pushrods are straight, without excess wear, and their oil passages are free and clear (ie, you can see down through the pushrods with no obstructions)

My first thought is that you have the wrong lifters in there.

I recall there are a variety of GM lifters that will fit in the lifter bore, but the oil hole on some of the lifters is the wrong height. If you have hydraulic lifters, this problem often (but not always) causes noisy lifters