Wee pixie
You wont go non stop GLA-SFB without refuelling in the 757 unless the A/C is almost empty and you get favourable winds and you wont be getting off the plane until you get to SFB as any refuelling is done whilst you are on the A/C. SFB-GLA can be done without a fuel stop but again it depends on the tail winds and how loaded the plane is.
757/767?. GSM probably wont know until a few days before so prepair for either. If it the 76 it wont be full so you will probably get to spread out and could get 3 center seats to yourself to grab some sleep.
If the seating config hasnt been change on the 76 then the mid cabin (premium Ecc) has extra legroom, if you've booked extra legroom you'll probably go there.
Hope that helps