Another 273 to 318 swap questions

The headers are a problem with ALL small blocks, not just 360's. I have no good advice for you, but I believe IF you can score the rare 340 manifolds, for the 68/ later, you can make them work.

But you could certainly get a 360 in there and running with your present manifolds, get other issues sorted, and then take your time and research GOOD headers. I'd guess some sort of "shortie" is what you want. "Back in the day" it was fender exit headers.

If Charlie above can indeed supply a bushing, get one, unless you have a local guy and nearly free use of a lathe. It's a simple ring.

If you go 360, using your original torque converter, you'll also need to buy a special B&M flexplate designed to balance the 360. It is weighted to replace the weights that would be on a factory converter. About 80 bucks, I think. Make sure you get one for an LA and not a Magnum, which are unbalanced at different weights.