You need to buy an adjustable pushrod, which you'll put in place of the actual rod and that'll tell you what length you need, then you order the right length. I actually just did all that last night.
Things you need;
1. two adjustable pushrods.
2. a set of checking springs.
3. a machinist rule.
4. one old pushrod.
5. black dry erase marker.
Temp install your heads with old gaskets of the same thickness to the ones you will use.
Drop in your old pushrod and this will help determine which size adjustable pushrods you're going to have to buy.
You should be able to "eyeball" the length needed. (6" to 7", 7" to 8")
Once you have the correct length adjustable pushrod checkers and checking springs;
1. Replace valve springs with checking springs. Checking springs have reduced spring pressure which won't bend your adjustable pushrods.
2. Color in the tops of valve stems with marker, install adjustable pushrods and rockers.
At this time, you should check to ensure the rocker tip sits square over the valve stem and is centered as much as possible.
You should also check to ensure you have clearance from your retainers to the bottom of the rocker body.
You may experience pushrod chaffing on the cylinder head... you need to watch for this when you rotate the assembly!
Once you have the correct length pushrods and you build up the cylinder heads, double check that the new pushrods don't chaff the guide plates. You don't need guide plates when you do the push rod length check since the nuts get in the way. I removed the locking nuts on my adjustable pushrods... they just get in the way and aren't needed anyhow.
3. Rotate assembly a few times to make a wear pattern or witness mark on the valve stem.
4. Remove rockers and adjust pushrods to center and make the witness mark as thin as possible. You have the correct length when the rocker body is fairly square to the valve stem and the wear pattern is thin and mostly centered on the stem.