Differences in 904 sun shells

So, I had read somewhere that the starter was slightly moved on the 904 when compared to the 727. This is a 727 case with a 904 dust shield. So they moved it on the 727 because the converter was bigger? Ok, but all small block auto’s have 130 or 131 tooth ring gear and 10 tooth starter gear. So the share the same tooth per inch of circumference. So they share the same ring gear dia? and if ALL that is true why oh why will my 727 B&M dual pattern flex plate possibly hit the starter when mounted to the 904?? Can’t wait to see what moving the starter up 1/2” does to my original 340 exhaust manifold clearance!