When i last checked the rules for improved production, you could only cut around where the gear stick entered the cabin, & you could only make it just big enough for the box. No complete fabrication of transmission tunnels. I know the standard SR20 box is a big bastard, because i still have the one that came with the SR20 i bought in 1994. One of your other options is to try for a Navara 5 speed & change the front half of the gearbox case to the sr20's. If you've only got a manual tunnel it's a bit of a bugger, but a bit of massaging with a hammer might get you enough room. You will definitely have to cut off the standard mounts.
I agree with grabbing the Maddat Gearbox crossmember, & using captive nuts in the floor.