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Torchlight's suggestion to use the Cadillac accesory drive will give you the most compact drive, but they're fairly hard to find and usually pretty spendy.

A F body Camaro front drive will get you back a ways too, but the ac hits the frame rail on an AD truck.

The Vette drive will get it back another half inch or so from the Camaro, but the Vette guys seem to be pretty proud of their parts too.

Another option is the radiator. The GMC radiator for the AD trucks sits further inside the support than the Chevy does to accommodate the longer GMC engine. It has a gooseneck filler tube to allow it to sit further into the support.

The brackets on the side of an AD radiator look like they could be modified to allow the radiator to sit in the support even further to give you a little more room.

You can always modify the firewall to let the engine slide back a little more. I've seen steel valve covers cut up and welded into the firewall to give some clearance to the LS heads. Saves a lot of metal forming, and I wouldn't have known they were cut from a valve cover unless they'd told me.

I moved my engine back almost 4" with a custom firewall. I wanted to use an original gas pedal, but it looks like I'll have to fab something custom because the engine having been moved back. Ditto with the brake and clutch linkage. The factory brake pedal now hits the back header tube, so I have to modify that too.

It's like whack-a-mole You solve one issue and two more pop up