Dual exhaust

dual exhaust

With the size of your engine, you will be better off going with a 2 inch exhaust, the larger size will cause you to lose bottom end power. You should also run an H or X pipe in the exhaust system as well, to improve the scavenging, and this will also improve bottom end power without hurting your top end.

even with a 350, unless you really warm up that engine, 2 inch exhaust wont hurt you any.

I put 2.5 inch dual exhaust with an H pipe, on a 302 in a 70 Ranchero several years ago, and it killed my bottom end. what I did to correct that situation was to get another set of header extention cones, and put them inside the ones that were on my exhaust system. The cone I installed necked the exhaust down from the 3 inch collector to 2 inch. With this inside the 2.5 exhaust, I got my bottom end back.
An added benefit to the cone inside the cone, was that it made an anti reversion outlet, so exhaust pulses couldnt rebound back up to the exhaust ports.

You will notice that this tech is being used with a lot of headers now, only on the individual runners.

FWIW, I doubt that you will find a muffler shop that can mandrel bend pipe. The equipment is extremely expensive and takes a lot of floor space.
That being said, lot of people believe that compression bending is not good for their car. While compression bending does reduce the diameter of tubing at the bends, most engines don't need a perfectly bent exhaust system. Real race cars that spend all of their lives above 6000RPM might need some wazoo system, but most of us are driving on the street, and the cost of mandrel bent parts FAR outweighs any benefit in power that might be seen by compression bending the pipes...
And (steps on soapbox), having been in the performance exhaust business for a long time, I am convinced that there are an awful lot of guys running around with a system that is way too large for their vehicle. No way does anyone need a 3" dual exhaust system for a 350 or 383 small block...It is a waste of money...I gurantee you that the driver could not t ell the dsifference in performance if I swapped in a 2 1/2" system when he wasn't looking...
On your 267, I would certainly not recommend anything over 2 1/4"