If I build an L36 Engine w/ rods, pistons, valvetrain, etc., what would theoretically be the HP limit of the stock block and/or crank?

 

Assuming you're keeping it FWD, I wouldn't worry at all about what the engine can handle. The stock bottom end can handle way more than the 4t65 ever will, especially now that the GMR parts are gone. Plenty of people have done 600++whp on bone stock bottom ends, and many end up selling or parting out - mainly due to the trans or lack of traction.

To answer your question more directly, I don't recall ever seeing a stock 3800 s2/s3 block or crank that broke from too much power. They always get destroyed by something else - spun bearing, piston frags, valve drops, etc.

As someone who went down the road of building a bottom end for a 3800 - I would strongly suggest against it. Just pick up a reasonable mileage L67 bottom and let it go. If you're to the point of actually needing rods / pistons / crank / etc, you should strongly consider another powertrain. No sense in building a 1k hp engine when the trans will struggle to stay together with half that.