on my prior boat I had a Supercharge 383 with big block headers CMI. I had a metal flange/plate that would bolt to my small block head and on the other side big block bolt pattern to bolt my headers I'm doing another small block project where can I buy that adaptor .
thanks in advanced

CMI makes small block exhaust if that helps.

Any decent machine shop can make one for you.
Just take a small block gasket a big block gasket
and tell/show them what you need.
I have found in the past I can get stuff made by a good machinist for less than it cost to buy off the shelf stuff from most aftermarket performance places and the fit is better since its made the way I want it..

i dont see how that would work,the 2 center exhaust ports on a small block are side by side,big block are spaced 4 to 5 inches apart,a adaptor plate wouldnt work unless you had 2 flanges for each and tubes between them and would have to be water cooled..the only way possible it would work is if you used a ls style or nascar head with 4 seperate ports.