ticking at high rpm

My freshly rebuilt 318 started a lifter tick at hihgway speeds that goes away when at city street cruising speed. It takes about 30 to 40 seconds for the ticking to subside after the rpms drop.. I checked oil pressure yesterday and was reading about 25 at idle and 35-40 peak when at high rpm. dont have a tach..
Bad lifter? Oil pump? any thoughts?
The motor runs with a lumpy idle, but not sure if it is supposed to. I have a comp 268 cam and 10.5 to 1 pistons with the block decked and 302 heads. Cylinders bored 40 over.
Dan