If and when you do get them, it would be great if you could eyeball the up with a US car to see if they might work.

In the meantime I'm going to try these shorties. I've had to face the reality I don't have the time or space to do up custom headers. This ought to be a significant improvement for my setup.

The one test I'm familiar with was done by Mopar Muscle on 360 with hp peak at 4500
Crate Motor Exhaust Test

Even that engine had torque improvement as well as hp gain with the Hedman shorties. A higher reving engine, or higher flowing port situation should benefit more. My engine's hp peak is closer to 5500 rpm and could use a bit of help lower in the range too.