I don't want to mess this up but have Uberti 1860 Army with a conversion cylinder for .45 Colt from Cimarron. I currently have HS-6 powder and True Flight. Bullets are Speer JHP 250gr. I'm thinking about 6gr of True Flight. Greatly appreciate some experienced input.

I don't know that powder but I would be sure to load light.  The Colt front loaders are an inherently weak design.  This became very apparent when Colt began to make that design for metal cartridges.  That's why they immediately copied Remington (The top strap frame which completely enclosed the cylinder)

I am not familiar with suitable charges for those powder, however I highly recommend picking up plain lead (swaged or soft cast) projectiles. The steel in replica percussion revolvers is pretty soft.

Desperado Cowboy Bullets (cowboybullets.com) sells suitable projectiles. I haven't used their .45s but plenty of their bullets for loading .32 S&W Long, .38-40, and .44-40.

A typical .45 Colt load for use in a conversion cylinder would be 250 grain lead bullet with 7.0 - 8.0 grains of Unique or Universal.

Why is soft lead preferable to jacketed?

Less pressure with the lead loads.

Thanks all

It will also put less wear on the bore. As I mentioned above the replicas are made from soft steel.