MSP, 63gr H4198 is max for the Ruger data with the 300gr Sierra HP in the 2006 Hodgdon Annual, Win brass and CCI 200 primers, COL of 2.525" @50kcup, Hodgdon got 2532fps in their 24" test barrel with that load.
FWIW, I get 2425fps with the 300gr Nosler PP with 56.2gr H4198, Rem nickel brass, Rem 9½ primer, COL 2.533" crimped with Lee FCD, in my 22" Handi.
The Hodgdon 2006 shows an overlap for the 300gr Sierra with H4198 tween levergun and Ruger levels, levergun starts at 55gr up to 60gr Max @40kcup, Ruger data starts at 59.5gr to a max of 63gr @50kcup. That's not a typo, if it is, it's theirs, 10kcup increase in only 3 grs seems a lot, but that's what it reads. I started at 50gr and worked up in ½gr increments until I found good accuracy, never had any pressure signs, all round ejected just fine, but each rifle is different, what works in mine may not in yours.
I believe you'll probably not want to go to that level without some recoil prevention in the light Handi, mine has the cinn lam stock set and lead filled pipe in the stock bolt hole along with a Limbsaver recoil pad. I also wear a Past magnum shoulder pad and use a high rest so my body is upright and can move back with the recoil.
Be safe and have a Happy New Year!!
Tim