It sounds like you've gotten some mismatched pushrods. There are at least three lengths of stovebolt pushrods, depending on which engine they're in, and the design of the valve lifters. 216's have a different length pushrod (shorter) than 235's, and there are two different lengths of 235/261 pushrods, depending on the height of the valve lifters. Older 235's with "milk can" lifters have shorter pushrods than later models with more conventional-shaped lifters.
There's an old shade tree mechanic's trick of putting the adjusting screw lock nut UNDER the rocker arm instead of on top that I've seen done when short pushrods are used in an engine that needs long ones.
Jerry
