I'm missing something. .
(<--- that's me)
Why not just turn the OD rather than messing around with this?
Trepan tools can take lot of time to get right and are not very fast cutting.
Generally they are avoided if possible. Not to say I dislike them as I make many thousands a year for various uses.
The outside does not need to be radius, a primary and secondary flat works just fine. One just has the clear the heel at the bottom of the tool in the cut radius. A quick sketch in CAD or even MS paint shows where and angle.
Do you have a standard carbide insert holder for OD work?
I know cutting less material is less total "work/HP/metalcutting" done but sometimes making a special tool is more time spent than a brute force attack of turning all into chips.
8 to 5.5 is a inch an a quarter per side at only one inch thick.
KISS.
Bob