Get Track of the Wolfs plans drawn from an Original 65 (IIRC) cal. Purdey Sporting the stock design is excellent for heavy recoil.
I have a 10 pound rifle that shoots a .662 ball, its about 40-50 grains lighter than a 69 caliber would shoot.
It has an 80" twist and never causes a problem with any load I care to shoot. It only has .008 deep grooves.
But the barrel is leaded screw stock and I have stopped shooting it pending getting a 4150 Gun Barrel quality steel barrel from Jim McLemore with a .695 bore. It will need a different ball but I have a .672" mould that may work.
If you are truly looking for a barrel he is working on tooling now but is a one man craftsman and things get done when they are done.
These are EXCELLENT barrels that he fires for accuracy before they are shipped. He will not ship a barrel that is not accurate. I have a 50 and a 45 in heavy contours. The 50 is in a match rifle and is VERY accurate. His standard twist is a gain. He will not tell the gain. Said he spent years getting it right. I am really excited to get this barrel in and on the rifle. Though I know it will make more recoil.

I would not go slower than 80" maybe 96" at the very most in a 69 no matter what people may think.
These twists will easily produce 1600 fps and that all that is needed in a ball that weighs more than an ounce.

Dan