We have a fixture to make 84-5 headers, but there is very little demand, and a quantity of 1 is not cheap.
If we did 5 sets, they could be priced reasonably, but it would require they be pre-sold for this to happen. The price then would be close to the AZGN/TA stainless performance headers.
Since the driver side header is the same as 86-7, that header would be an improved 3 into one design.
We would not use the stock-type flanges for the crossover, they would be stainless band clamps like we do on most headers, including 86-7 GN.