JammerUSN

Ok so I just installed a new head on my 92 Toyota pickup.
found coolant in the oil and opted to purchase a complete set with gaskets and new head.
spent the weekend installing it all
Followed all the specs and sequences in the Haynes manual.
Engine is timed and valves adjusted at operating temp.
truck idles nice and revs nice, but is now gutless barely breaks 40 mph 3rd gear, forget trying to get to 4th and fifth without a long stretch
So I pulled it back in and found it has a bad exhaust leak at the manifold gasket. I noticed the rivets for the heat shield seem to be in between the manifold and the head not allowing it to seal
so my question are. Is there a better gasket to use and could this leak cause it to lose that much power ?