Von,
I just added the 6-2-1 headers and a 2.5 inch MSA exhaust to my 76 the other day. Other than having, in my opinion, quality control problems with the pieces themselves, the car runs fine. My car had no EGR system on it at all so there was no question of anything extra to have to hook up.
Here is a run down on the problems I encountered with the installation:
The lower section of headers would not completely slide together with the upper half. If you tried to slip a pipe over its mate one at a time they fit fine but together at the same time and they would only slip maybe an inch together. I solved the problem by cutting off about an inch and a quarter from the top section downpipes and then using my mig to just weld up the lower to upper sections after I slipped them together.
The 2.5 inch kit was a pain from the get go. First, the bell ends had not been expanded far enough, but luckily I have an expander and a torch so it was easy enough to put in the expander, apply a bit of heat and it opened up for the sections to slip together. Then when I put the first two sections on the end of the header instead of the pipe being in that recess by the diff where the stock pipes sit these ended up midway between the recess and the drivers side wheel. I took and cut out a wedge of the 2.5 pipe near the header, pushed the pipe until it was in the recess and tacked it together. The rest of the installation went fine and I welded up all the seams.
More than likely your mechanic took one look at the FI plumbing and does not want to mess with it. It is really not that bad a job. My car runs great and sounds even better. Have fun.........
Charles