I used to race a '72 240Z. Just to let you know, I don't think that Datsun made a 240 in '74. That was the first year of the 260Z and it was a POS (in terms of reliability). If I remember correctly, they had some big problems with the carburators. My '72 was a pain because I built everything up on it from suspension, tranny, rearend and 6 different engines. Webers were a real pain in the ass because they went out of synch so easily.
The Mazda had its share of problems with the water seals on the rotary. Until the 13B engione came out, they were all irratic (in terms of reliability). The Gen 1 RX-7's were real lightweight and rockets if you knew whatto do to them. All you had to do was add a great foam air cleaner and a set of headers/exhaust and that car would go like a real rocket.
A shop in West Los Angeles used to import 13B engines from Japan to transplant into US Mazda's. There was a pretty big difference between the older rotary and the 13B.
Both of these were fun cars at the track. The Z car tended to be nose heavy and plow when the ground got wet. The 510's actually handled much better than the Z cars. The Mazda was pretty balanced in it's handling (especially after you swapped out the shocks, springs and sway bars.
Just my $.02.
Brad