FYI, the Mexican blocks are not higher nickel, they were desirable in the 70's-80's because they were the only 302 block to have thicker main bearing caps, similar to the 289 Hi-Po blocks.
They looked beefier, so most people considered them to be stronger, but many machine shops (at least in my area) prefer the roller cam 5.0 blocks, which actually do have higher nickel iron compared to late 70's, early 80's blocks.
To answer the OP, there is nothing inherently wrong with a 302 compared to the 351w, other than being a smaller and lighter block. As others have said, it will make a ton of power with the right parts. The 351w will stay together while making more power simply because it has 49 more c.i. and the block itself is stronger with thicker main webs and bigger main bearings. However, I can't go along with a blanket statement like "a 306 is a great engine." Being a 306 means the engine has been rebuilt, and as is the case with any rebuilt engine, how good it is depends entirely on the quality of the parts used and skill of rebuilder/machiner. You can have a 306 built from a '77 core by boring the block and stuffing it with stock replacement .030 over cast pistons and stock crank, rods, bearings, with little to no care in the machine work. Try adding power parts to that and it's going to cost you. If the rebuild started with a roller block, included quality machining, forged pistons, ARP rod bolts, etc., you can build power with no worries. It boils down to two things, what is the seller telling you the engine has in it, and do you trust/believe him?
