There's three basic accessory drives for the LS (not including the transverse engines)
Corvette - shortest but the alternator may hit the inner fender well of an AD
Fbody Camaro - about 3/4" longer than a Corvette, but low mounted alternator may hit the frame rail.
Truck/SUV - fits between the frame rails of an AD but is about 1.5" longer than the Corvette.
Ideally what you'd like to have is the length of the Corvette drive but in the truck/SUV configuration to fit between the frame rails. These guys say they can do that with aftermarket parts-
https://lsbrackets.com/ There's a few other things you can do to buy yourself some more room.
A GMC radiator will fit about 3/4" further into the radiator support than a Chevy, but you'll have to figure out what to do with the cross brace rods. Apicella Designs (
https://apicelladesigns.com/) recently came out with a radiator for the Chevy ADs that pushes it further into the support, but I imagine they're pretty pricy.
The second thing is to push the engine as far against the stock firewall as possible. You could consider a bump-out on the firewall, at least on the passenger side, but you need to make it big enough so you can take off a valve cover if need be
For my '49 I took out the factory firewall entirely to make more room for a truck factory accessory drive, fan shroud and fan - it was a lot of work.
