I cant give you the numbers for stock sc valves size,but i will explain some things involved.
If you install a high lift cam.as (a example) .575 lift or higher you will need to use longer stemed valves.
so when the cam lob lift pushes the valve open, the spring pack around the valves will not coil bind.You
need to keep .050 + space between the coils of the spring. To fix, you machine the head directly under the
spring to lower the spring pack,and,or you install valve spring collor ,that has a deeper dish in the center.this will raise
the top of the spring pack to give the coils more space.so they do not coil bind.More rocker arm raito will add to
the coils binding.To find out if you will have coil bind you put a dial indicator afix to the top of the head with the dial indicator plunger on top of the valve stem.Then push down on the valve,at the same time watching the dial indicator
until you reach .575+.050=.625 .If no binding that part of spring pack set up is done.then you take it back apart and
measure for spring pressures you want.to due this you need a small type arbor press with dial indicator,Press the spring down .625 .the press will have a gauge to measure the wt. of the pressure that you apply this will measure spring pack pressure.When buying your valves you can get them with the same dia as stock from tip to valve head,or buy taper valves ,from the valve tip, to where stem just exit the valve guide,inside the port, it can be a smaller dia.for more flow.
ROLLER ROCKERS, If you purchase rocker with a higher rocker arm ratio ,this will push your valves open farther.If you
want more valve lift,and if you do not want to go to the trouble of installing a cam .
If you do the above you may have to change pushrod Legnth.You can find what lg.you need with a adjustable pushrod.
WHY WELD PORTS, EXHAUST OR INTAKE.Head ports drawn up on paper may look great.But buy the time the factory casts thousons of them the dies become worn and miss line,with inperfections,from air to ect.
When a head porter gets your heads to port .He will inspect them, and fine were the valve seat has been press in on the intake. There can be a large void in the port that meets the valve seat.If you look inside of the port (intake side) you can
see the bottom edge of the valve seat sticking out.The porter will remove the valveseat, and fill the void with weld,press the vavleseat back in ,and make a cut so the inside dia.of the vavleseat matches the inside dia.of the port .So the air can
pass from the port into the combustion chamber,with out hitting the back of the valveseat.Porters do build up welds,so
they can cut or grind back down to change shapes,fill voids ,raise the floor or top of port,or to taper port to raise air speeds.