273 to 340 swap

If you had used the '89-'92 '308 heads(Pre-Magnum 360 Roller), then exhaust wouldn't be a problem, either, but I see you used 'J' heads.
The '308 heads used the smaller exhaust ports like a 273 has, which makes using the stock '64-'66 273 manifolds more feasable.

Sure your exhaust flow won't be terrific (largest pipes that'll fit, along with good mufflers, will help immensely), but on a street car torque is the mover, and there'd be plenty of that.

Mark.