If I'm understanding you correctly, you want to install a Lyman 66 sight on a rifle.
First WHAT rifle?
In most cases you do need to disassemble the action at least part way to insure you don't drill into working parts, and to keep metal drill shavings out of the action.
If it's a Marlin made up to the 1990's, it may already be drilled and tapped for a receiver sight.
If so, there's two filler screws on the rear of the receiver.
Newer Marlin's like the 39-AS are only drilled for a top mount telescope and you can buy receiver sights that mount on top using those holes.
If it's a Marlin you want to install the sight on, field strip the rifle by separating the two receiver halves and remove the bolt. That's all that's needed.
Considering a Marlin is now a very expensive rifle, I don't think I'd be attempting to use a hand drill to drill the holes. It's just to easy to tilt the drill and drill slanted holes, or allow the drill to drift off-center.
You can't UN-drill a bad hole.