Long legs/shot torso modern trail bike fit

Hiya

I’m looking to buy a 2021 Giant trance x 29er. On Giants I’ve always struggled to find a good fit. I’m just shy of 6 foot with long legs short torso and longish arms. So on the Giant sizing i sit on the medium and large cusp.

When i tried both the medium I had to put the seat all the way back on the seat post head and I stilll felt like I was sitting off tthe back of of seat by about an inch. I also had my knees hitting the bars occasions. But the shop guy said when I tried the large he felt i was over reaching and looked like I was pulling my shoulder forward when turning. …but otherwise I felt comfortable on large and loved that I felt naturally over the centre of the bike when climbing. While on the medium I had to pull myself forward on the steepish climb.

Shop guy says he could but a 10mm longer stem on the medium but will that fix sitting my tendency to sit on the an inch off the back of saddle wth it jammed all th way back on the rails ??? I gather I’m not alone in my long leg/short torso dilemma
Shop guy has no bias to sell either bike out of self interest so i think he is really trying to do the right thing.

HELP !!!