Longer seatpost, too much stress on Reynolds 520 frame?

Just got a KHS Flite 100 size 55. It's a bit on the smaller side for me and when the seat tube shows the maximum line in the "peeping hole" (horizontal with the top tube) on the frame I'd need an extra 4-5 cm to be comfortable. The current seatpost is 26.8, 22.5 cm long.

Would buying a longer seatpost (40cm or so) and raising the seatpost 5cm still put undue stress on the frame or will it be fine?

It's too small for you.