Amount of preload is insignificant until you start floating valves, then it gets serious real fast. If you want to play it safe then less is more. 1/4-1/2 turn is sufficient. A few thousandths is all that's required.
I use between 1/8 turn to 1/2 turn preload after getting zero lash with adjustable rockers and pushrod set up
3/8x24 adjusters equal right at .0416 so I use .042 per turn, 7/16x20 equal .050 per turn
I like to see between .012 to .025 clearance under the lifter retainer to the pushrod cups cold on iron heads and a little bit more with aluminum heads, say another .006 deeper, with the intake manifold off
if you have non adjustable rocker arms order a checking pushrod and use it to determine which length non adjustable pushrods you will need.
I would check at least two cylinders, one on each side, and both valves to see if your going to need different length pushrods on one side or not and also if the intake valves need a different length pushrod than the exhaust valves do.