Question for you guys with LS motors

[64shortbed]

I put this here because it is for the 64 GMC. When you put your LS/Vortec motors in and used the 63-66 factory engine mounts did you have to move them forward to the front? I have a 350 in it right now and the mounts are in the factory spots. But looking at the frame they can be moved forward looks like about an inch. I'm asking because i have a 06 5.3L DBW with 4L60E i'm going to be putting in my 64 GMC. The other question i have is did you have to modify the floor of you pickup (low hump) to make clearence for the trans? And last question is just for looks. My harness is a stock harness i'm cutting down to make stand alone harness. Where or how did you mount the computer? And ideas or help? Thanks ahead of time for the help.