Do you still have the A13 engine in your 1974? It was the most unique Datsun engine, only used for one year, and only in one territory (North America). Rated at 75 hp (about 45 SAE J2723 horsepower)

The best engine swap may be the KA24DE out of a 240SX -- 156 hp is more than double the original!
* Fits in the B210 like it was made there
* You can use a 510 reversed crossmember, it's just like a 510 swap
* The 240SX 5-speed fits in the tunnel just have cut the sheet metal in the shifter area