Dirk, In general yes, the diameter of the primary pipes will affect peak torque value and what rpm that peak torque occurs at. Exhaust gas velocity increases as diameter decreases, but flow rate(cfm) decreases as dia decreases. For street driven rpms most advise 1.75-1.875 inch pipes is fine for big cube motors. There are software packages the pros use to simulate best header designs when it comes to racing.
Here is the owner of Sanderson Headers take on affects with data for a small block engine. The 533 cubic inch monster has plenty of torque in any case whether 1.75 or 2.0 inch primarys are used. There are other ref out there as well
http://www.sandersonheaders.com/tech_get_technical.php