The old Leupolds are highly desirable, for a reason. If you have one that’s a good start. I use a fixed 2x on a Freedom Arms .454 and it is a very effective combo.
For a red dot, the Aimpoint Micro is the king for durability on a handgun. I really like the red dot on a hunting handgun, my Jack Huntington .500 Linebaugh Super Redhawk has a T1 on a Weigand mount and it’s been excellent.
That isn’t to say there aren’t other good, durable red dots, I just prefer closed emitters on hunting handguns. I’ve use Aimpoints since my first trip to Afghanistan in 2002 and still use them wherever applicable to this day, they’re phenomenal.
Big bore handgun conversions are my favorite hobby, I’ve tried about everything under the sun at this point. The singleactions.com forums have a huge amount of information and it’s very well moderated like this forum. Certainly worth a look if you haven’t been over here before.