Because you don't seem to be familiar with making gaskets, I'm copying something I wrote some time ago on how I make them. I very seldom buy ready-made gaskets - the gasket paper is cheap and the gasket sets aren't. And my "plan ahead" quotient isn't very good.
Getting screws out: I use a 3/8 air impact with a #3 Phillips bit set for fairly soft impact torque and let it rattle away at the screw until it comes loose. Setting for high torque has a way of demolishing the screw heads right away as does putting a lot of pressure on it with a leveraged screw driver (like with a Vice-Grip pliers attached to the shank).
For case cover gaskets you don’t have to buy ready-made gaskets at all – they’re easily made, and I make extras when I’m making them for future use. The ones I make seem also to be more durable than those supplied in gasket kits.
Always start by making sure the cover for which you’re making a gasket is flat. Setting the sealing surface down on a piece of emery paper placed on a perfectly flat surface and moving the cover around on it will polish the high points. They’ll have to be filed flat. Use a single cut file – much smoother surface, easier to file things perfectly flat. High points will be around the screw holes.
I don’t remember the thickness of the gasket paper (haven’t had to make them for a while), but I get a roll of gasket paper from the local auto shop. I set the cover for which I need a gasket down upon it and trace the outline of the cover, then mark all the screw holes with a scribe, poking small holes at several points on the circumference of the hole. Taking the cover off the gasket material, I use a compass (tool for drawing circles) set to the width I want the gasket to be and follow the outside line with the point, using the pencil to draw a line for the inside cut. I use a large gasket punch (any small round object will do, but I can see where I’m setting the punch through the hole in the side) set over the screw hole marks to draw the cut line around the screw holes. I use a smaller gasket punch aligned with the scribe pricks to punch out the screw holes.
Carefully cut along the inside line with a single edge razor blade or (better) a craft knife, being especially careful around the screw holes. I radius the turn into cutting around the holes, and I actually draw a line for that radius before cutting. Cut those first: the gasket material is more rigid then, making sharp curves easier, and it’ll save you from the error of cutting off the mounting hole part of the gasket.
Save the cutout piece for smaller gaskets, often best made by marking the outline of the covers then re-constructing the damaged gasket to fit that outline and marking the inner borders of the gasket.
Where possible, I cut the outside of the gasket with a pair of sheet metal shears leaving a considerable margin of perhaps ¼ to 3/8 of an inch. This gives me material with which I can hold and position the gasket, and – anyway – the method is easier than trying to cut precisely a large narrow gasket (although it’s necessary for a few things where the gasket can’t be trimmed after installation). I install the component with the gasket in place, tighten it down, and then trim the outside edge of the gasket to match the case with a single edge razor blade.
I tailor the gasket compounds to what I want the gasket to do. Consider whether you’ll want to take that cover off and replace it without any fuss or it isn’t going to came apart again ‘til you’re doing a complete motor rebuild and you’d really rather it didn’t leak in the meantime. Some things, like a side cover or valve cover I’ll use a “permanent” gasket seal on one side of the gasket and a “removable” seal on the other to avoid one half of the gasket coming off with the cover and the other half trying to stay with the case.
Incidentally, I’ve found using a cork/rubber compound gasket material, even though it’s marginally thicker, works nicely for the alternator rotor cover. I only use gasket compound on one side of that, and get no leaks.
Most covers aren’t highly critical about the thickness of the gasket, but a few are. The covers over the camshaft ends determine camshaft end play and have to have gaskets of a precise thickness.
Over tightening screws probably won't stop the covers from leaking and may well be the reason they leak in the first place. It can also damage the threads in the motor case, which can jam the screws in place making them very hard to remove - and compromising their ability to be screwed back in. Aluminum is pretty soft stuff, and none of the screws have to be any tighter than you can get them with an ordinary screwdriver.