You can still use the bg (phase II?) manifold on the bh (phase III) block, what you need to do is make up an adapter plate, the same as what you would do for a bellhousing adaptor. If you have both of the manifolds off you will be able to trace out them onto a piece of paper to figure out what your adaptor is going to look like.
You will need to get it cut out of alloy, more than likely getting it laser cut, which alot of people around here will tell you where to go depending on where you are.
You can either supply a 1:1 tracing of what you want to the profiling company, or you can/get someone to draw it up as a .dxf for you to give to the profilers.
What area are you from?
that is another way actuley hadnt put any thort into it!, will try to source a complete manifold and bg engine loom and can messure up i tryed geting a gasket for the heads but repco list the same gasket for both motors lol