I have installed Volvo hs6 su's to my Z. They seem to be very close to the original round tops. The bore is very close to the original. And the carbs fit straight in to old intakemanifold. I ordered needles and repair kits to my carbs from England(I live in Finland) Now they seem to be workinkg o.k exept that the car hesitates a litle when I go fast in to the corners. I got the idea of volvo carbs couple of years ago. I did some resurch and found out that there is a firm in australia that does the same thing. They sell rebuild Volvo su's that are modified to fit datsun. Check out www.sumidel.com