I was digging around to answer the OP's question for myself, as well as the operating range for the manifold. I found the answer in Offenhauser's 1981 or 1987 catalog.
I have the
5488, which to the best of my understanding is identical to the
5615 except for the bolt flange angle. I cut on mine, and got some spherical washers to fit it to the 1966 and later LA heads.
Here are some screenshots from the catalog. Page five and twenty-one of the PDF of the catalog.
http://offenhauser.co/wp-content/uploads/2018/04/Offenhauser-Catalog-1987.pdf
If I am reading their tables right it seems to me, depending on your cam and carburetion that torque is between 3,000 and 7,000 and horse power between 2,500 and 7,500. All the more reason to go with a solid cam instead of the hydraulic roller that came with my 1991 LA360.
Looks like AFB carbs only. I will be buying a pair of the Eddie
AVS2 carbs in 500cfm size when they finally release them for sale.
In reference to post #11, I am going to be adding material to the outsides of the runners so I can port match to my 308 heads (or the Trick Flow 190's when they come out this fall and if I scrounge up the money).