The -8A80 and -8A90 are 4 into 1 and include an offset pan. The -8980 and -8990 are 4-2-1. There also were a -5950, -5980, and -5990, which are 4-2-1 stainless. The last two digits are the muffler material and 50/80/90 are stainless/titanium/carbon. There also was a larger diameter exhaust that I think is different from these. I've seen it referred to as the AMA spec system. It might be 4-2-1.
The only ones I found here in OZ were for a K3-4 and K7-8. The K7-8 can be used if you swap the oil pan and pick up from the K7-8 to your K5-6 (apparently). I haven't done it yet but I have the parts to do it. The K7-8 collector can be rotated and has the spring hooks to do it. This changes the power delivery supposedly, but I haven't investigated this.