sounds like it may be normal, bump the idle up like others said, 500 is too low. I like to keep the idle a tad high on old engines to ensure oil reaches every last part of the intended path. I'd go over 750rpms. Don't know what trans you are running, you need to stay below a certain point with auto trannies so they don't start stalling the engine when you are stopped and in drive. 1300rpm idle would not work
Had a guy bring an older malibu to the shop with a new high perf 350 in it, he complained about stalling at every damn red light, and he kept adjusting the idle up. Must have idled 1200-1300. I dropped it to 800 and his stalling problem disappeared. Go figure