Which is the best way to run the accelerator cable out through the firewall up to the carbs? I had it up between the seal and tin in front of the impeller.

There's a hole in the tin on the passenger side that it normally routes through for FI, and I've used that routing with two different styles of carb linkage successfully. You usually need to make some kind of bracket for the end of the cable sheath that attaches to the fan housing, or the oil filler tower, or one of the case through bolts, depending on the linkage you're using.