Official Difference btween the 396 and 402 BB Because you asked.

[70cst]

The official difference between the 396 and 402 BB according to the Feb, 1970 Chevrolet Dealers Memo

The top power option for all Chevrolets Series 1030 trucks (except the Blazer) is a big 400 V8 with 310 horsepower. It has the same power output as the 396 V8 it replaces, but it has a number of significant differences, such as:
1. Gasketless, tapered-seat spark plugs and conforming heads.
2. Throwaway-type oil filters.
3. Two-step automatic choke for better engine warm up
characteristics.

The increase in displacement was accomplished by enlarging the cylinder bore rather than lengthening the stroke. This, plus the use of a low profile inlet manifolds, keeps overall height low for more room under the hood. And remember, with this engine, you get a lot more horses under the hood than competition can offer. Here’s the way the top engine of each major make compares:
Biggest V8 & Max. Gross H.P. CHEV. 400/310 FORD 390/255 DODGE 383/258 IHC 392/236

Now you know the rest of the real story...