Hey Steve...
I gotta ask a couple questions...and please don't think I'm being a smart a$$ because I'm not. Have you been loading BP cartridges long? Are you using a drop tube to dispense the powder into the case? Is your Pyrodex load 53 grs. by weight or by volume? If it's by weight that's a good load as Pyro was developed as a volume for volume replacement for black and is some lighter. Also, if you don't have these two books, get them. The fist is by Venturino and Garbe and is their spiral bound book on loading black powder cartridges, I can't remember the exact name of it. The second is by Paul Matthews and is "Loading the Black Powder Cartridge". Their methods are a bit different but the results are equal.
Part of the issue may be the rifle itself. Also, if you have lube on the target that means it also isn't mixed in with the fouling. Specks of lube on the target isn't a bad thing in and of itself but it still means the lube isn't mixing with the fouling in the barrel and that's where you need it. The 405 gr. bullet obviously doesn't have the lube capacity of the heavier bullets, or most don't. When using real black are you blow tubing your barrel? Obviously this isn't something one wants to do while hunting and I don't, but, when developing a load it can help.
I cast all my lead 25-1. This is a compromise I came up with and it works well in all my rifles, altho some prefer a softer mix of 30-1 and some a harder mix of 20-1. There isn't enough difference in my rifles for me to cast up separate batches for each rifle, at least at my level of competition. The 20-1 alloy you mentioned from Cabela's should work admirably.
The BHN (Brinnel Hardness Number) of pure lead is 5.0, wheelweight is generally 8.9. A 20-1 alloy is probably about 6.0- 6.5 as it is softer than wheelweight. I'm guessing at 20-1 as I don't have a hardness tester. If someone does have a tester I'd be pleased to know what 20-1 really is.
I'm impressed you got such good accuracy out of 70 grs. of Pyrodex. Usually a load that compressed shoots more like a shotgun.
As far as giving up on a mold I quit pretty quick. With real black powder, which is all I shoot, once you have the essentials of the load chain down that changes very little. I've shot a lot of old black powder cartridges and have a pretty good idea what will work and what won't. The first thing I do if a rifle isn't shooting as I think it should is change bullets. I prefer a bullet with a bore riding nose and I like the nose to enter the rifling a few thousandths upon loading. Somthing one is cautioned NOT to do with smokeless powder but often encouraged to do with black. Have you tried seating your bullet out farther? Of course that depends on how your rifle is throated, if at all (most of those old Italian replicas weren't throated at all). There is a way to determine this dimension but you can't do it with a gas-checked bullet. That was a bit of rambling on my part.
If you're going to order some bullets from Cabela's try the Lyman bullet, number 457125. It's a 520 gr. (from my mold at 25-1) round nose and a wonderful bore riding bullet. I have had good success with it out to 400 yards altho after that I think the round nose is a detriment as it won't shoot with my RCBS 500 BP mold at greater distances, which isn't a bore rider but it has a long shank.
I have rambled quite enough. If you can't tell, I love shooting those old "buffler" rifles. I've been doing it about as long as that IAB has been laying around your house!!....hehehe! Good luck and let me know how it works out for you.
Vic