Thank you for your input, ggzilla.
I spoke with Bruce at Z therapy a couple of minutes ago. Bruce told me that I could lose power in the low end should I use dual carburetors that are larger than 38 mm. I believe that I would prefer 38 mm carbs to the larger Roadster carbs which have been offered for sale to me by the510keeper, who, I assume, is very knowledgeable about his carbs. I would like to see pictures of his carbs and do more research before I decide whether or not to purchase them. The carbs may be just fine for my application. Bruce told me that the SSS manifold is a two bolt manifold and that it should work with my 1978 620 and its l20b motor.
You appear to be correct about the term "SU" when used to describe Hitachi dual carbs. The term is a misnomer. The term is not used to confuse. The term is used to describe the similarity of dual Hitachi carbs to their British counterparts - the SU carburetors made by Skinners Union. According to Wikipedia, SU carberetors were originally designed and patented by George Herbert Skinner in 1905. There are a large number of British car enthusiasts out there, and I am one of them. In general, British car lovers are familiar with the look of SU carbs. These carbs appear in MG's, Austin Healeys, Jaguars, etc. British SU's have distictively shaped float bowls. I believe they are called "domes" by Z Therapy. I am a newbie to Ratsun. I am not that familiar with all of the terms used to describe Datsuns nowadays. Last month If the owner of a 510 told me, "I have a dime with dual Hitachis.", I would have been confused. Had the owner of the dime told me last month, "I have a Datsun 510 with dual SU's." I would have been able to picture his car in my mind. I read somewhere that dual Hitachi carbs are superior to British-made SU carbs. I don't know about that. I liked the dual SU's on my 1962 Mk II Sprite. The SU's on my Sprite had a small pump. I could could prime the carbs if I hadn't started the engine for a couple of days. I liked that. I do not believe that the dual Hitachi's have a priming pump.