EBay the headers. Chalk it up to learning.
Get yourself either an inexpensive pair of 67-69 Firebird 4-tube headers from Hedman (PY used to sell them for $59.95) and modify a couple of tubes, or a pair of the "long branch" 67-69 Firebird cast iron manifolds.
Mad Dog also makes excellent headers for your exact application:
http://www.maddogheaders.net/Catalog.html
