Help with removing head on 43F

[Pawlie]

Hello All,

Undertaking a complete tear down to replace the fifth gear and to freshen up the top end. The head has never been disassembled. 1986 xt600 43f

I've got to the point where I need to remove the four main bolts that hold the head on. The ones inside with the copper washers underneath. I've applied quite a bit of force to them and am worried if I use more, they will break.
Is it ok to apply a bit of heat to free them up? Will this do any good? The manual says no heat as it might warp the head.

Any hard won knowledge from thoes that have already been down this road?
The acorn nuts in the next step look scary as well!!!

Many thanks,

Pawlie