I am going to put my 2 cents in #1 if you buy cylinder heads complete you can expect them to be put together with cheap crap ( example ) they will tell you they used a good quality valve but becareful what are the springs retainers and keepers had guy walk in shop with nice dart heads he bought complete for his set up. You got it broke a spring, smacked piston, damaged head and broke A guide and thats not the worst. Dummie that built his short block did not restrick oil to top and he pushed out a lifter lost oil presure and now needs crank reground and 1 rod. So beware of the bargain - YOU GET WHAT YOU PAY FOR. These where ebay heads from some speed shop in cal. Thumper cams ?? sound great driving though steak n shake but don't make the power for the buck centerline to low try to stay 110-112 and for nitrous need to open exhaust valve more and longer. I did this with 1.8 rocker on exhuast and 1.73 on intakes - my new bullit will just use 1.73 - still waiting on the heads should be here by end of month