[frame sizing] GT Zaskar 2005 - M or L ?

Hello!

I'm around 182 cm (~6 feet) tall. At the moment I ride 18" hardtail frame
which I find slightly too short (the top tube is 55 cm (21.7 inch) long, I
don't know the exact horizontal TT measurement, but its makeshift
measurement showed 58 cm (22,83 inch)).

My inseam is 86 cm (33,86 inch). I need a frame for cross coutry races and
marathons. What GT zaskar size would you recommend: M or L? Or mayby I
should look for some 19" frame? Please help.

All the best,

Lechu