I can say this with fairly good athority. That is not a 88 chevy set of heads. they are early heads. 88 5.7 heads have the centerbolts for the valve covers. So you could be all over the place with your timing.Also did you check the timing gears to make sure you have the key in the proper slot if there are more than one??? Backfireing could be a bad intake gasket. Now I'll be quiet since I have confused even me.