Nitro 450, great project. This is going to be wordy.

I have stuck with the 20ga for deer exclusively and had great luck. I also had a similar project that is in phase two (better platform) and will hopefully be finished by fall as well.
A couple of discoveries so you don't have to reinvent the...

Slower twist is definately better with a full bore slug. I had Bauska (when they were in business) cut a barrel with a 1-70". .615 bore, .625 groove. It shoots 3" Brenekes into 2" at 100yds. It did better with Lightfields.

Fast twist barrels lead. Badly. I modified some molds, tried factory and never got full bore slugs to shoot well in a fast twist barrel. If you follow the chart, your barrel twist in 20ga is optimum with a 900gr bullet. Those were my experiences.

For fast twist barrels
I have had great luck with the winchester platinum tip slugs in a lightweight H&R with the quick twist. The load replicates a .454 Casull (+100fps) and the bullet has excellent expansion. Makes it a true 150yd deer rifle as that is as far as I have shot it.

Lastly keep things supersonic. I mentioned the 3" mag above. The same slug in 2 3/4 would group into 1" at 50yds, went subsonic at 75, and grouped into 8" at 100. I upped the speed with the mag and the problems stopped.
Never tried round balls (but will in the future.)A number of years back I shot a doe with my 58 flint at 140yds. Ball went through both sides, deer went to freezer. I can only imagine the .62 being that much better.

Best of luck with your project/

Tom