There may not be a "reason" why its leaking,but it is leaking. The adapter could be warped the gasket bad etc...get some black RTV and put a light coat on the mounting surfaces,and make sure to torque everything down snug and that should solve your problems.
No worries,so you ran a line from the PCV port on the carb to a manifold vacuum source? If so you need to remove this line and just cap the PCV port on the carb and the manifold vacuum source.
Here you go: http://www.pentastarparts.com.au/sunshop/index.php?l=product_detail&p=490
As far as making manifolds. If you have a foundry,and a lot of money the sky is the limit.
Yep,look for a matched set of carter bbs,or holley 1920's,or you can have the mounting pads drilled out and use BBD's,Holley 2280's,or even dual webers.