If the recoil of your weapon or your abuse on the weapon calls for the screws being upgraded to 8-40 you had better have one bad ass scope cause I sure that the optic will fail well before the screws do.
The 8-40 screw are gonna have bigger heads than the hole in the base was desgined for so you have two options 1-make the hole bigger 2-make the screw head smaller. Brownell's offers an 8-40 scew that uses the T-15 size torx head wich it better for that screw size because the T-20 wrench will snap off the screw , where the T-15 will snap off the tip of the wrench and keep you from a major head ache. The 8-40 heads are bigger though they have to have wider heads so that they will have the proper bearing surface in relation to the bigger hole so your best bet would be to have the scope base holes reamed to fit.
check here : SIGHT SCREW COUNTERBORE at Brownells
they are expensive little tools. if you need a square bottom hole you can use a 1/4" carbide 4 flute end mill to make the counterbore , BUT , you have to make sure that the base is dead center of the hole and held very rigid before you try to sink the mill into it , and use the shortest mill you can find so that it won't walk on you.