I adopted a tier system for Burden in my house rules.

Burden 1 SoWs
Burden 2 Post Ships/Corvettes
Burden 3 32 gun Frigates
Burden 4 38 gun Heavy Frigates
Burden 5 3rd Rates (Constitution stayed here)
Burden 6 1st Rates

I standardized all classes as I felt that slight changes in broadsides didn't need representation.
I treat Temeraires as heavy 3rds and Bellonas as Light 3rds, using the same stats for all matching models.

The 4 vs. 3 broadside in Amazons an Concordes I attribute to the difference in length. The Amazon's shorter length gives a more focused broadside.