Had the same problem with my '75 Super. No decent exhaust. The only difference with the FI exhaust, besides the extra smog crud and the one exhaust hole in the apron, is, they reroute the other two cylinders exhaust back into the heater boxes for added heat. And yes it does heat!
If you don't have any DEQ issues, then either go to "J" pipes if you don't need the heat or try buying a set of used or new '72 "style" heater boxes and then you can use the earlier style exhaust. (My wife has a '72 Super, so that was just as good a year as any for me.) You will have to cut some new holes for the exhaust (I bought the twin pipe style) unless you go with one of the "quiet pack" style that goes under the apron.
If your '77 engine bay is the same as my '75 and you do go to Duel Carbs be aware that you WILL have to take the Carb/ Intakes off when you pull/replace the motor.
As far as i know I THINK someone mentioned on one of the other forums that a 1640cc is all the farther the stock FI could go??
'72 and '75 Super Beetle's