351 heads screwing stuff up?

[vetteeatr]

I bought this beast mustang but it has 351 windsor senior heads on it. I cant figure what the deal is with it lately. A horrible thing happneed so i had to rebuild the engine but the enigne then started making a thunking noise. or sometihng more like a hard ticking. It seems to be consistent with my RPM's as well. I replaced all of my pushrods and also have harland sharpe rollar rocker arms for 302 or 351(is there a diffrence?) I als had to replace one lifter. Im guessing its the lifter but i dont know why. would a stock lifter screw something up ? I dont know if the other lifters were aftermarket or not. Please help me out guys its becoming quite aggravating havin to tear my enigne apart every other day. Thanks