Did a couple of measurements on my RPM heads and I got about .300 thickness between pushrod and port, does that sound about right? I am using a 3/8"-7/16" tapered pushrod and am getting just a little contact. I also measured the side to side width of the pushrod hole and got .500. Is increasing the hole .050 considered reasonable? Actually, it appears I am just making the hole round instead of oblong. How much clearance is actually necessary?

Do you mean .030 between pushrod and port? Can't imagine contact with .300 clearance unless your pushrods are made of rubber.

Opinions vary but for myself run clearance tight. Once I can slide a piece of paper between the pushrod and (everything) I consider it done.

I would run it awhile, 5 to 15 minutes and then pull the intake pushrods to see if they are rubbing, shiny, polished, in some spots

The .300 is the thickness of the head between the pushrod hole and the intake port, thus is the amount of meat I have to work with to cut. I wanted to open the pushrod hole up to the point where I have .020 clearance. Since the pushrods are tapered the higher they go, the bigger they are. When I was checking the scrub on the rocker arms, I noticed I got to a point in rolling the engine over by hand that it stopped. I have over .250 of piston to valve clearance, so checked the push rods and it appears I had a couple that were touching.

Man you must be running some crazy spring pressure to need those pushrods.

Actually no, pretty mild Isky 8005A. After trying a couple different pushrods and comparing lift numbers, I was surprised to see the amount of deflection, so my theory is to run as big as I can, plus these large pushrods are actually super cheap as NASCAR leftovers and you can usually find them in .025 variables, so no need for custom pushrods. When you move over to ball on ball pushrods you also eliminate the "kick out" from ball adjuster and can actually run longer pushrods. Jack your shafts up along with thicker, longer ball to ball pushrods and you going to get the most out of your cam, as you have eliminated deflection, kick out and scrub loss.

If your seeing deflection with a 3/8 rod, something is wrong. With those springs, you shouldn't. It's possible the larger rod is binding a bit on the head, causing the lift to be better at the valve, but it won't like it once it's running!

I'm not following the binding causing a higher lift at the valve-can you explain?

The deflection was from a 5/16 pushrod.

Agree 100%

I had a set of custom made Smith Bros. 3/8 pushrods that were supposed to be .120 wall 3/8 diameter, one of them broke in my test mule motor on the local Studka engine dyno at 5200 RPM. That pushrod broke into two pieces (it had a metal flaw in it where it boke) and bent the one next to it and falling out allowing that pair of roller lifters to pop out of the lifter bores allowing the broken one to get into the bore and gauge the cam lobe
When I took those two bad ones back to them they told me that those pushrods were .083 wall, not the .120 wall I had ordered and paid for They replace the two broken ones for no charge.
I ordered a set of identical Manton .120 wall x 3/8 diameter and made five more pulls on the dyno with the Smith Bros. pushrods and them swap in the Manton's with no other changes and they made 8 HP more at peak RPM (6700 or maybe 6800 RPM, CRS now , they gained more power from the start to the finish of all pulls after that
My last Hi Po bracket motor had Manton7/16x.145 wall pushrods on the exhaust valves and 3/8 to 7/16 single tapered pushrods on the intake valves to help add some more clearances on them to the head up high near the valve cover gasket surface

Well, that was generous of them /s
They didn't replace the entire set with .120 that you paid for??
I got my last set of pushrods from Manton, guess I will continue buying them there!

They didn't offer to replace all of them probably because they didn't have any 3/8x.120 wall stock
I didn't think to ask what the price differences were between those two walls sizes That was the last pushrods I bought from them
My revenge is to tell anyone who ask about Smith Bros. pushrods to tell them Mantons are a lot better at a better price
I was told several years back that Manton doesn't keep any shelf stock, they custom make each set

As the rocker gets pushed up, if the rod binds on the head it CAN come out of the PR socket in the rocker arm and actually push the rocker further open than normal because it's sliding out of the socket and therefore gaining lift as it does so. Not saying that it is doing that, but it's worth a look.