Pontiac Repair: pontiac 421, how much oil?

Question
HI, I own a 1969 GTO convertible that has a 1965 Pontiac 421 in it, verified by casting number reference. My question: how do I determine how much oil to put in?
The motor has an aftermarket oil pan, I do not know brand, anywhere from 6-8 quart pan. The motor was professionally built many years ago by previous owner, the dipstick is not stock. I have been driving the car using the dipstick that came in it, but it takes way to much oil to get a reading. The dipstick is 17 1/2" from the tip to the end  by the handle where you pull it out of motor.
This is a tough one to figure out. I cannot find any diagrams or specs on the dipstick, also does an aftermarket oil pan change anything?
Thank you! I will surely donate with a valid response and I really appreciate your help. Shane, Cape Cod Mass.

Answer
Hi Shane:

Well first off that car is the same age as I am. that aside. not knowing what size the oil pan is obviously an issue. if it has the stock pit stick tube and indicator in it I would fill it accordingly to the dip stick. if it has an extra deep sump in the pan you have no idea if the oil pump has a pick up tube to match the pan. I would hope it does if not too little oil will starve the pump. not knowing what's in there if it has a windage tray or not to keep the oil away from the crank when it's running is the issue with over filling it. if you put too much oil in it will foam and cause leaks. I will stick to my original though fill it till it shows on the indictor if it is indeed original that way it will have a proper level on the bottom of the crank and you will not starve the pump for oil.  the issue with that is knowing if it's original or not. I'm not trying to double talk I'm just typing as I'm thinking. That is the best Idea/though I have for you sorry. as for oil use a good brand like Castrol or Valvoline no knock off store brand. Hope that helps