J series width dimension below the valve covers

I’m considering a J series engine for a swap in a Datsun Roadster. I believe the height is approx 28″ and width about 21.25 (valve cover to valve cover). Before I get a mockup engine (or better) I’d like to know about the width below that 21.25 in the area down where the exhaust port(s) are. If this works out I’d be using a monoport variant so if someone has a dimensionfor that monoport cover, that would be the most helpful. If the 21.25 is indeed correct and someone can just tell me fore example that the monoport cover is say 1.5″ more inward than the outside edge of the valve cover then that would tell me what I need to know. Thanks so much in advance to everyone for the helpful info. Jim