Hey Lynn,
I used a standard spool type mount but had to modify my stock crossmember to do it. The "block/cube" type mount in mine was in poor condition so I thought it made sense to make the change. The spool type allows you to use a multitude of mounts far greater in numbers than the stock type.
The standard procedure for the fitment has been to dimple the tunnel at the points where the two case bolts make contact. Another consideration is where the speedometer sender sits near the floor. You can dent/dimple the floorpan to get the GV to sit higher but then you'll need the 90 degree adapter so the speedo cable isn't pressed against the floor, causing misalignment and erratic readings. I had a 90 degree adapter but the threaded nose cracked. Gear Vendors was out of stock so I had to lower the end of the case to allow use of the standard speedo housing.
If you are heading into the winter with some down time, it may be worth it to call Gear Vendors and inquire about that 90 degree housing. Ask for Mike. They actually are pretty helpful even when you don't buy new from them. I was impressed enough when I was installing mine that I sent them several extra speedometer gears that I had once they told me that they had customers waiting for them. The GV uses a speedo gear from a GM turbo 400 trans. I pulled NUMEROUS ones from local junkyards trying to find the correct tooth count for my car. If you still have my number, feel free to bend my ear about this install.