Hello, Would a good quality (say Smith Bros chrome moly) 5/16 pushrod be ok for a eddy bb head with 377 open load bee hive spring? This will be a street motor not to see over 6200rpm. Hydraulic roller cam. Or should I go with the 3/8s?

well I can tell you I have Manton 5/16x .118 wall push rods on the intake side of my small block running 700# open at 7500 with 0 problems but they are very short at less than 7.00 long

yes, We used em on a SB circle burner hitting 6300 & no prob & these were stock ones and Smith bros are the top of the line

Most performance pushrods seem to have .080"+ walls. Not sure how much is needed with incrementally heavier springs, but they will be able to tell you how thick you need to go.
Length and rpm's are also factors.

Rule of thumb: Always pick the larger diameter pushrod. Always. The problem with pushrods is that they are in column loading and the ends allow bending in any direction so they can go crazy. They deflect easily and there goes valvetrain stability.

Pushrod stiffness goes up to the increase in diameter CUBED, IIRC. Comp Cams has done research and found greater valvetrain stability with larger diameter pushrods even when they were heavier.

"But I'm using super-trick Chrome-moly-zenon steel! Doesn't that make a difference?" NO. The only thing your super-trick steel will do is not permanently deform due to the bending. In other words, the yield strength is higher. But the AMOUNT of bending or deflection is controlled by the physical dimensions of the part and by the Young's modulus or spring constant of the steel and that does not change appreciably with steel composition. So if you use low-strength pushrods and they deflect they may stay bent while the super-trick ones will deflect the same amount but come back to original shape. You won't have a clue they are bending unless you se a Spintron to see what happens at high rpm. As you can't change the spring constant the intelligent thing to do is change the physical dimensions in the direction of greater stiffness; that is, increasing the diameter.

End of lecture.
R.