Several considerations suggest the replacement of the stock 964 exhaust. If the G50 transmission is used in this application, it may be possible to retain the stock headers. If the G50 transmission were selected, use of the stock 964 system would require relocation of the SC oil tank due to interference with the 964 final muffler, or alternatively, elimination of the final muffler. Secondly, there is a problem is the interference of the passenger side final muffler with the SC oil tank. The decision to retain the 915 transaxle creates another interference problem for the stock exhaust. The transmission shifter fork lever arm occupies the space which the right side header needs to cross over to the left. Of course, another thing which must be considered in a choice of exhaust system in the case of streetable conversions is your local emission control laws. I have not investigated a way to include a catalytic converter, but also don't know that one is an absolute requirement to meet an exhaust emissions test.
One requirement for my conversion was to retain the capability for interior heat. Thus far, two heat-capable systems have been tried. For the first, I chose a pair of SSI heat exchangers. This choice will require some modification to the SSI heater boxes, especially on the right side due to interference with the 964 chain tensioner. The first muffler tried was a dual inlet, dual outlet, manufactured by Triad. This was located on the used market, and can best be described as looking like 3 domestic glass-paks. Not a lot will be said about this muffler except that it proved unsatisfactory due to a resonant condition making clear thinking inside the car a challenge. The next muffler was a stock '74 dual inlet single outlet. Use of this muffler with the SSIs (and the stock engine sheet metal) requires either 1) reshaping of the muffler, or 2) extending each exhaust header with a short fabricated extension, or 3) lengthening of each header by splicing in a short section of the 2" section of the header. In order to retain the original configuration of the SSIs, number 2 was chosen. Use of the SSIs, in order to retain cabin heat, still required some creative metal duct work to the heater box input to provide a workable heater system. Further investigation after installation of the SSIs suggested that the primary header pipe diameter of the SSIs (1.5") may be too restrictive for the 3.6 liter engine to achieve its horsepower potential in this conversion. I discovered that B & B Fabrication makes a system which is especially designed for a 964 conversion. The header/heat exchangers have 964 heater box inputs and SC heater box outputs. This system is a bolt-on to the installed 964 and can be specified to have a muffler with dual outlets, or if desired, with a single outlet on the left side as the SC. The primary tubing is available with either 1 5/8" or 1 3/4" sizing; I chose the 1 3/4". (See photo on "Summary "page)
Other header-only solutions may be possible in the absence of a requirement for cabin heat.