replaced cracked heads because it was missing: still misses backfires and has no power: always had problems missing when at 1/2 tank of gas I had this done because I know very little about it: what should i look for to correct this problem:

I'd check fuel delivery; check the posts in this forum, there are several about overheating fuel pumps and clogged fuel filters. That may be your problem also.

are all the spark plug wires in the same place that there supposed to be?

I'd think that if spark plugs wires were in the wrong places, that there would be missing regardless of how full the gas tank was.