sorry I can't help you myself with small block stuff but You can try shooting an e-mail to some of these guys, one of 'em should have something for you, or know someone who can get one in the St. Louis/St. Charles area.
The selection with aluminum,std.273/318 port size and square bore in one sentence is VERY limited,however.....weiand and offy did offer them years ago and with a little looking you will come across one. Here's a list of small block manifolds you may want to put in your favorites/bookmark section. http://www.thomsclan.com/users/sthoms/cars/intakes/LA%20intakes.htm The eddy D4B or LD4B were decent manifolds,but are hard to find. What other plans do you have for your engine.Is it totally stock now?