Do you have reason to think your dipstick is off? Oil capacity can vary depending on oil pan used, and maybe the existence of an oil cooler?????
My guess would be 5 because that was common for engines of that era. So if 5 shows up proper on your dipstick, that's probably correct unless you have reason to believe otherwise.
As I recall, there was an issue with 87 chassis year 454 dipsticks being off. The cheap remedy was to do a complete oil and filter change, add the recommended amount of oil per the manual, run the engine to fill the filter, shut it off and let the oil drain back into the sump. Then, use a triangular file to mark a new FULL line on the dipstick.
As I recall, there was an issue with 87 chassis year 454 dipsticks being off. .
When you're dealing with a vehicle that old there's always the question of whether it is the original dipstick, which is why I asked if there was any concern about it. I have an 89 vehicle that I've owned since new, but that is hardly common.