Common problem with gummed or rusted valves. Best way to deal with that is to oil the valve stems, then tap them to make sure they move easily, a rubber hammer or 2 x 4 works well. The result as you found out is bent pushrods or broken rocker arms. Never burn antique fuel, which will gum up the valves and also cause that problem.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.