The factory .451 balls should fit the Pietta Remingtons. The factory .451 balls usually do get swagged into the chamber when rammed, and wads or over powder cards can be placed under the ball for additional protection against a chain fire.
And
your chambers can be precisely measured to determine their exact specifications.
There's no doubt that the .454 balls will fit tighter.
I don't know about the size of the balls that the .451 mold will cast. It depends on the purity of the lead and the exact specifications of the mold. Molds are often slightly oversized, but can also be slightly undersized as well.
There are ways to fix undersized molds, and also if using impure lead the balls will end up being slightly oversized.
If loading off the frame using a table top loading press, then using .454 balls or larger doesn't require much of any extra effort.
But if loading with the frame mounted loading lever than the amount of loading effort is a matter of personal choice.
As well, there may or may not be a slight accuracy difference between the 2 different diameter balls. That's up to you to decide which size ball is preferred.
And don't forget to buy a revolver nipple wrench, or make one yourself as shown in this thread:
http://www.thehighroad.org/showthread.php?t=578275
