Need to know the stock exhaust system pipe diameters for my stock 1974 L18 620 so I can get my stainless system built. I don't want to make any modifications and want the new system same as original equipment stock.

I thought the entire system was 1.5" but it's looking like the exhaust to muffler clamp called out in carpartsmanual (20643-B5000) is 45mm or 1.75 inches.

The tailpipe diameter is more difficult because it came with the muffler - no clamp. There was a mickey mouse hanger clamp at the end of the tailpipe (20641-B5000) but I can't find what diameter it was.

Measuring the existing pipes is obviously going to give you the sizes you need. In the USA, you won't likely find the metric sizes originally used. The 1.75" is close enough to 45mm to be used as an alternative match to original equipment. Using a caliper to measure the existing pipe diameters is the easiest way to find what you're looking for and then just match up with what is commercially available to build out your system. If you don't have access to a caliper, wrap a wire around the pipe you want to measure and then mark the wire length of wire it takes to wrap around the circumference of the pipe. Measure the marked length of wire on a ruler and then divide that number by 3.14 to get the diameter.