FWIW - I pushed my engine back about 3.5" into the firewall and put a 'doghouse' in the cab. I'm 6'3" and it didn't seem to take enough room out of the cab to matter any.
Here's the new firewall cleco'd in
The doghouse on the inside of the cab looks like this..
Your engine sits quite a bit lower than mine. I jacked my engine up to get the headers to fit better
A LS truck motor in an AD truck is a little bit of 10# in a 5# bag. In addition to moving the engine back, you can get another 1.5" or so in front of the engine with an accessory drive that uses the Corvette dampner. I've seen a few trucks that ditch the cross bracing on the radiator support and push the radiator into the support to gain another inch or so.