To build a top end properly is a very skilled job. As you have had the head skimmed the valves will be closer to the bottom land of the pistons. You must check that the valves don't hit the pistons by measuring the gap between piston and valve, (I use plasticene) which must not be less than 40thou. Put 3 pea size blobs across the cutaway in the pistons, build top end, don't bother with pushrod tubes, but torque everything down to correct spec, adjust valve gap, turn engine over once and take head off and measure the thickness of the plasticene by cutting it in pieces with a scalpel. If all is ok fine, but if not you will have to remove metal from the cutaway until you have the right clearance. I use a Dremmel to do this.
Also don't forget to make sure the fitted valve spring length is correct in the head, measurement must be as per workshop manual. It probably won't be, so you will have to make shims to fit under the bottom cups to rectify the problem.
Once all this is perfect, then you can look at the pushrod oil seals. Fit seals so that you have a 60 thou crush. Not enough and you will have oil leaks.
