Valve stems seizing on the valve guides are a likely cause - try to get them well lubricated soon. Unleaded gas is icky stuff........Sometimes use of the wrong intake bolt length can get you in a bind as some of them will stick down into the pushrod openings. Check it out with a flashlight.The lost pushrod can't do much damage where it is - they can only lie longitudinally in the galley. Many old engines are found to have a few dead ones in there.
Which pushrods got used in the rebuild? If they are the "skinnies" (small diameter/solid section), I would suggest trying to find a set of the tubular parts for replacements. Tubulars got used after 1957 and they are the ones that you will find as replacement parts - but the length can be off so be picky.