The 6" reamer works fine on the #1 through #4 mains, but the drill chuck gets in the way of the block at the rear seal retainer area, so that is where a drill bit extension comes in handy to clear the block wall at the #5 main.
If you are a decent welder, you can also weld a rod to the reamer to make it longer and not have to spend $50+ on an extended length reamer.

Drilling works too, done it several times and only broke one drill bit, but after using the reamer, it just seems to do the job easier and quicker, with a cleaner finish, less burrs and such where it breaks through, and smoother walls inside the reamed hole.