340/mag360 exhaust manifolds

yeah, that's it!, now how much difference in performance do you think that would make over the 318 one I have?, what I would have to do is use the 1980 318 truck one I have on the p/s to match diameter of exhaust, could this one be machined bigger? and how much do you want for it if it would work?, this must be a 69 340 d/s one that you have I presume?,thanks again for your help.