I think older versions had riser on right side , newer versions we're on left . I think I have that correct . I used older on one side and newer on the other side to end up with no riser . The non riser manifold seems to be longer to compensate for the missing riser to match up with the front exh pipe . Stuff seemed to fit ok and looks cleaner I think .
I cracked a stock heat riser once when I tried to mill out the center shaft too fast, if you do this make sure to go slow and use cutting fluid (if using a mill). I think I was using a big 2 1/4" rotabroach to try and get a slightly bigger diameter exit as well . . . that was probably a mistake!
If you were driving the car in winter where it gets very cold like 10* out the heat riser might help warm up the engine faster. That is all it was really used for to get temps up faster. Probably more of a restriction in the exhaust than helping to warm up the engine. The one Lark I have I wired it open permanently. If I wanted any performance out of the engine I would take it out and replace with exhaust pipe or get some headers for it. Plus it would have a nice sound with a more stock type muffler on it if you do not want a lot of noise when using headers.
