ORIGINAL: RiverOtter

I've got a Lyman Great Plains rifle (sidelock percussion) in .50 cal. It's got the 1:60" twist, and my current load is 67gr of FFF Goex behind a .15 patch and .490 hornady roundball. Here's the thing: the manual states that max load for roundball is 70gr of FFF or 90gr FF, and I can't remember what the max loads for conicals were offhand. At anything up to 75 yards it's pretty flat and centered, but at 100 it's about 3 inches low and 6 inches to the left.

I'm going to hunt with this load all of next week because I know where it's hitting and I was successful with it last year. After that I'm going to change out the sights to the Lyman 16 AML/37 ML setup, which has a white dot front sight (better visability in low light) and folding adjustable rear sight. I figure as long as I have to sight it in all over again I may as well try to improve on the accuracy to tighten things up. Here's my question: What other loads might I try considering the slow twist? I've read that 1:60" or longer is pretty much ideal for roundball, but there are some conicals that will work well. Any proven loads out there?