I am sure that Daryl will be chiming in on this.

I am curious about what action you want to use.
It is easy enough (albeit time consuming) to monoblock a single shot.

That being said, when I was working on my 20ga I went with a 66 twist. It was meant for Lightfield and Brenneke slugs which it did very will with 100yd groups around 2" or less. With the velocity you are looking at, that twist will give you some flexibility. A bit fast for round balls but capable of stabilizing slugs.
Personally, I don't think that you need any more than a round ball (around 350gr). My 58 has shot through every deer and pig that I have ever taken, so I would think that would be the case for the 62 as well.

As far as dimensions, I matched my barrel to the cases I planned on loading with. I measured the inside of a Brenneke hull (I am thinking .625) and went with .005 rifling. I am not sure about the forcing cone.
You will need a piloted reamer because of the rifling although you could probably chamber it with a forcing cone reamer and finish the rim recess with a regular reamer.

You may want to reach out to Bob Hoyt in Pennsylvania.

2379 Mt Hope Rd, Farfield PA
717-642-6696

He rifled a few barrels for me and has a good reputation.

Good shooting.

Tom