273 upgrade to 4bbl?
here you go - Offenhauser Low Profile Single Quad manifold part# 5613. From the 2006 Performance Automotive Warehouse (PAW) catalog. This is a really old design as there are drilled and threaded carb mount holes for a Carter WCFB carb (used on many early Mopar 4bbl engines). Offenhauser unlike any of its competitors made (and still make) small runs of any of their manifolds for decades after they were introduced. It might be possible to order one of these new. I am pretty sure that this is a single plane (360 degree) manifold since all the runners are at the same level. If you want me to e-mail you the scan PM me your e-mail address - that way you can see more detail in the picture.
here you go - Offenhauser Low Profile Single Quad manifold part# 5613. From the 2006 Performance Automotive Warehouse (PAW) catalog. This is a really old design as there are drilled and threaded carb mount holes for a Carter WCFB carb (used on many early Mopar 4bbl engines). Offenhauser unlike any of its competitors made (and still make) small runs of any of their manifolds for decades after they were introduced. It might be possible to order one of these new. I am pretty sure that this is a single plane (360 degree) manifold since all the runners are at the same level. If you want me to e-mail you the scan PM me your e-mail address - that way you can see more detail in the picture.