Because they look virtually identical, 351W engines (so named because they are built in the Windsor engine plant) are often mistakenly identified as a 289 or 302. In fact, the 351W is based on an entirely different block that is stouter and has greater deck height than the 289/302 (289/302 is 8.206 inches while the 351 is 9.480 inches for '69-70 and 9.503 for '71-current). However, both series of engines use the same bore spacing and the bolt patterns are the same, so 289/302/351 heads will interchange. But, because the increased deck height puts the heads further apart, the 351W requires a unique intake manifold.
Other differences between these similar-looking engines are main bearing diameters (289/302s measure 2.25 inches, 351Ws are 3 inches); slightly larger rod journals (2.1232 inches compared to 2.311 inches), and the the fact that the 351W uses a longer connecting rod (5.956 inches compared to 5.1550 inches for the 289 and 5.0900 inches for the 302).
When swapping parts between these engines, the most confusion surrounds the camshafts. Camshafts will interchange, but the 351W has a different firing order (1-3-7-2-6-5-4-8) compared to the 289/302 (1-5-4-2-6-3-7-8). Compounding the situation, the '82-and-later 302 HO engines use the 351W firing order.hope this helps.