Hey,
Doug's will fit, but be prepared to change some things. My '65 Barracuda has a 360, power steering, power brakes (hydroboost), and a 4-speed OD gearbox. To make it all fit I had to modify the steering column and use a Flaming River steering shaft joint instead of the bulky stock coupler. The stock power steering box was replaced with a Borgeson unit because it's much smaller. I also went with a hydraulic throwout bearing on the gearbox. Along the way I had fiddled with a modified Z-bar, slave-cylinder getup that actuated the clutch fork (Chuck's classic cars), different clutch forks and pivots, all sorts of failed experiments. I even tried the heralded 340 exhaust manifolds. In the end a hydraulic clutch / throwout bearing provides a great deal of simplicity and clearance once it's in place.
So while you might just be thinking exhaust, if you go with headers be prepared to modify many other things. I wish I knew all of this stuff before I installed my headers. But now that it's all in place everything is fine.
You might want to think twice about cutting up your car and re-arranging your suspension just to attach headers. But as you know, it's the curse of a small engine bay.
In retrospect, I wish I would have just stuck with the original 273 and manifolds.
