Wow! I haven't seen an Offy Dual-Port in years!
I ran one on the street back in the early '70s on my old '69 Dart 340 Swinger and was very pleased with the performance. It was kind of a 'super' dual-plane intake manifold.
I was impressed with the engineering design concept of the small primary runners for high-velocity intake charges and the isolated larger secondary runners for a cooler intake charge when the carb secondaries opened up. Being smaller than a full intake runner also kept the cooler secondary charge at a high-velocity.
My 340 was built to the extent that it required a Holley 850 double-pumper to keep it fed or it ran out of gas on the top end. I was also running a full reverse-pattern manual-shift 727 and 4.89 gears - all on the SoCal streets. I loved picking on big-block Chevelles!
Sorry, I don't have any 1/4-mile times or speeds for you. All I can tell you is it worked and it ran strong.
The funny part is that I walked in to J&N Speed Center in Whittier, CA, with the intent of buying an Edelbrock single-plane ('Tarantula' I think it was called) and walked out with the Offy Dual-Port 360 because they didn't have the Edelbrock in stock. If I remember correctly, I sold the Offy for $25 at a swapmeet at OCIR when I decided to take my old Dart off the street and build it strictly for the strip.