.
Something else that a few others have pointed out, I measured my LOP using the crease of the elbow to pad of my bent trigger finger and it is 16 inches…
That’s a good rough measurement for a traditional hold, but it varies based on shooting style. Squaring up behind a rifle uses a shorter measurement. If someone only shot from one position it would be easy to figure out what works, but everyone uses a slightly different hold for sitting and standing. I get a chuckle out of guys trying to square up outside of prone.
You’ve been shooting rifles too short for you your entire life so it will take a little time to sort out what feels familiar vs what works best. There are many different pieces of metal hardware to extend the buttplate, or any length of spacer can be fabricated.
Scope placement must feel random when LOP is wrong, you don’t have a consistent cheek weld, or even an agreement to what amount of cheek weld is better, scope height isn’t correlated to cheek weld. You don’t only have long arms, but the distance from shoulder to eye is longer so the geometry of a standard stock is not doing you any favors. You’ll either need taller scope rings and higher cheek piece or lean the head farther forward.
Natural point of aim can help you sort much of this out. Every time you shoot close your eyes get the gun in a comfortable natural position and open - the scope should be at the far end of ideal and still provide a full field of view.
You’ll quickly run into disagreement between how much cheek weld is ideal. Without cheek weld the main way the head is indexed behind the scope isn’t there and it doesn’t take a great leap of faith to see how scope cuts will be more common with anything recoiling more than a 243. I’m a traditional shooter and don’t square up, and like a good amount of cheek weld, so when someone uses little or no cheek weld I simply remind myself if it works well on target there’s more than one way to skin a cat. With heavy recoiling rifles no cheek weld is a great way to get stitches from a scope cut.
Having said all that if I were in your position I’d get a stock with metal hardware allowing the cheek piece and buttplate to be adjusted. Short of a custom stock that’s going to provide a great way to sample all different combinations to figure out what’s ideal.