need a pic of master cylinder with manual setup

OK, you have the standard '73-'76 disc setup; that is good.

The only way I know to be sure on the bore diameter is to pull the snap ring out of the back, while making sure the piston does not pop out too far and measure the bore or piston. This is because I cannot tell you for sure if the piston's bore behind the snap ring is the same as the bore in front of the snap ring. Maybe someone else knows for sure.

BTW, the correct disc/drum MC will have a much larger rear 'bulge' in the cover than the front 'bulge', and a much longer rear reservoir to match the longer rear 'bulge'. If the front and rear bulges are the same size, then you have an MC for a drum/drum setup. Also, the disc/drum MC cast body is taller.

I know that the NAPA MC Part # that I listed will be right. Can you list the Autozone MC PN? My concern is that you said it was for a '70 model and that is very likely incorrect.