Will a 6 qt be enough?

The oiling system on a stock SB Mopar does need improvement such as you have done. Some aren't as bad as others. Maybe that's why some guys are lucky enough that theirs survive and others don't. I just did a 340 for a buddy last summer and I found the oil pump feed hole in the main cap was only 3/8". Should've been close to 1/2" if I'm not mistaken. Don't know what happened on his but it seems like they missed one last machine process when they built it. Or they used the wrong size drill bit. I opened it up to 1/2".

BTW: did you do any mods to help the windage tray drain back faster?