It's been so long ago all I remember was I shot 4oz of shot and 400grs of 1F. A square load is by volume, a equal amount of powder and shot. I have two old adjustable powder/shot dippers and the drams is shown on one side and the shot in oz on the other. They don't go to 3 or 4 oz, but one shows at 2oz 4.5 drams and the other 4.75drams of powder. So doubling that we get either 246 or 259 grs of 2f at 4 oz. Say 250 and if I add a bit for 1F, maybe 300. Looks like I was a bit high, but then it was a gun I helped make [ there were 3 of them, each of us did a different job making them ] and the barrels were quite thick. They were side hammer O/Us and they weighed around 20 lbs. The brother in-law who owned a machine shop made the barrels from thick seamless tubing, the neighbor made the locks, triggers, and all the hardware, and it was my job to get the wood and make some guns out of everything. If I shot directly overhead it would push me flat on my back. It wasn't so much of a kick, just a big push. I ended up choking mine by heating the end of the barrels and with a tapered swedge put about .008 in each barrel. At 60yds it would completely cover two newspapers side by side. And the geese coming in to decoys in farm fields dropped dead. I first tried buckshot but couldn't get good patterns so I went to #2 shot. I'd get the nipples out one way or the other. Even if you have to drill them out and re-tap them. A 1/4X28 is a common thread and nipples are easy to find with that size. A believe it's a #7 drill and the tap shouldn't cost you more than $20 and a couple of nipples and you're good to go. Go to Track of the Wolf or Dixie Gun Works for what you need.
www.circlefly.com has the wads you'll need. Good luck.
Threaded Mode