My guess is that you have over tightened the rocker arms to the valves do not close properly.
If you did not touch the rocker arms at all, perhaps you have mixed up the push rods or the new head gasket is thinner than the old one.
If ths engine has hydraulic lifters (it should), loosen up the rocker arms a bit, then slowly tighten the arms while rotating the pushrod with your thumb and index finger. When you feel a slight drag on the pushrod, rotate the rocker arm nut exactly 3/4 turn. This pre loads the lifter. Do this only on the base circle of the cam, when the valve is supposed to be closed.
This procedure is for the small block V8, but my Haynes manual says it is the same for the six.