If you are running low comp they should be well clear anyway, its high comp engines where you tend to get the most problems.
Place sum plasticine or mouldable wax on the highest part of the piston (if they arent flat), smear a small amount of oil on the head chamber, place the H/Gskt on, drop the head on, nip up the headbolts slightly, rotate the engine thru 2 revolutions, whip the head back off,slice through the squashed wax/plasticine in sections and mic up the thickness, try not to pull it off too hard (oo er!) as it will stretch and become thinner. (do this for all 4 to be on the safe side)
You are looking for around the 1mm mark, this allows for heat expansion and rod stretch at high rpm
Good Luck!
Oh...just to add, if you think the valves may come into contact, its the same principle but you will obviously need to put the cambelt on etc.... (solid followers would also be advisable to use as we all know the Hydraulic ones creep back in which will mean the valves wont be moving as much as if the engine were running!