I have a chunk of b body floor from a 69 road runner four speed car that I've saved because its a great template for cutting to do a conversion. Here is my question though. The factory obviously must have cut the floor before the crossmembers were in and I've read posts from people doing a cut for a four speed with the important warning about not slicing into the torsion bar cross member. So why is the hole so big in the first place? Obviously the trans and shifter aren't going into that space. Why not just cut the hole up to the crossmember and cal it a day?

Because the hole wasn't actually cut, but stamped. When the sheet metal is placed in the die, it will move around. To account for some of the inaccuracy, you make the hole a bit bigger. In reality, its only a small part of the hole that resides along the cross member. Figure a +/- 1/8" placement for mass production, which is probably pretty good for that era. You also need some additional room to get the assemble engine and transmission up into place. The factory used carts that were fixed on track and a conveyor system to hold the body and drop it down on cart. Not a lot of lateral movement, so again, to allow for some tolerances, you make the hole a bit bigger.

I would remove the floor steel from the welded on hump and then make your hole smaller than the hump in your floor around 1.5 to 3.0 inches inside the O.D of the hump outline

That would be too small on a b-body. The shifter does come up into the tunnel area and there needs to be space for the rods to move. I made a hole a bit too small and the 3/4 shift rod was hitting the edge of floor pan making shifting difficult. You can see the paint chipped on the pic. Put the hump on the floor pan trace around it. then add 1" to the inside and that'll get you close to factory. I used a Black and Decker saber saw. Easier to control and had a small kerf in case I ever want to weld floor back up.

I did a '69 Road Runner. Here are a few pix if that helps. Jeff