Wondering if this is just my thoughts or am I going nuts???
Of course you're going nuts
rafsob. You're a muzzleloader shooter aren't you?
Minnies are not likely to shot well in any gun not designed specifically to shoot them. Those guns have very shallow grooves and even then the Minnie must be sized for a close fit to the bore.
It's a shame the Great Plains bullets won't load easily in that .45 Seneca. That would be the perfect bullet for that gun. If all else fails, it might be worth it to slug your bore to determine its exact diameter, and order a custom sizing kit from Lee.
http://leeprecision.com/custom-lube-and-size-kit/
I wonder if it would be worth trying 180 grain or 200 grain .40 caliber XTPs in 45/40 sabots.