1994 F150 problem with new engine

We just replaced the 5.0L engine with a rebuilt long block. It ran good at first then started missing and using a lot of gas. The plugs looked fouled. We thought it may be an emmission controls problem and had that diagnosed. We were told the no 6 cylinder was low (20 lb comp). The mechanic said he thought it was cross firing and the piston is damaged. I found this hard to believe. Has anyone heard if this is possible on Fords especially within 50 miles on a new engine?

Upon removal of the valve cover we found both rocker arms for no 6 cylinder were loose - so much they were completely off the valves and push rods. They were adjusted good when we put it together, and it ran fine for the first few miles. How are the valve adjustments supposed to stay locked in? If we readjust them they will probably do the same thing since the adjuster bolts seem to turn easily.
Any help?

You have got yourselves a defective long block. They won't help you with your labor, but go back in time and get a diffferent long block.

Just to close this thread, we found all the rocker arm bolts on both sides were loose. They should have come from the rebuilder torqued to 24 ft lb. I have checked all torques of everything in the past on a rebuilt engine and almost checked them before we closed it up this time, but assumed all to be good. Thus the old saying about assuming....:uhoh: Anyway, now after a couple more hours work it seems to run fine. Just opening the valve covers is a lot more work than it used to be due to all the stuff they hang on top of an engine. Next time everything gets checked while its still open. Good advice to all.