I need to order some guide plates but don't know if I need the 5/16" or the 3/8". Anyone know? I know I could just pull the valve cover and look but asking you guys is easier at this point.

Nobody knows?

Order the 5/16"....it'd be odd that you'd have 3/8" pushrods. If your valve train has been upgraded that far to need 3/8" pushrods then it already has guide plates.

If it's stock and untouched why do feel you need guide plates with the factory self aligning rockers?

Thats all i needed to know. I got some stamped steel roller tip rockers for free from a friend and wanted to put them to use but since they werent self alining needed some guide plates. Thank you.

5/16" is stock sbc pushrod dia for all gen 1 sbc engines.

You do know to run guide plates you have to have the heads machined right?

The pressed in studs has to be pulled out, holes tapped, and bosses cut down, then install shouldered type screw in rocker studs along with the guide plates. All in all that's about $250-300

You can buy self aligning full roller rockers for for about that same price.