Just my two cents. I don't think it matters much which year for finding parts etc. There are all kinds of parts and options available for this sized block no matter what the year. I am currently having a 351W performance engine built for my 67 cougar. When I finally decided to go with the 351 we had to find a block. If you have a choice then good for you! I took what was available at the time (out of a 93 F-150).
I have heard that for strength the early years were the best (69-73?). These blocks had high nickel content or something like that which made them a very strong block. I think it all depends upon what you intend on doing with the engine. If you are going to race this engine at high revs then maybe you need to be particular about the block and seek out an early year model for its strength. However, if you are going to use this as a daily driver and occasional performance, any block in good condition will do.
In my opinion one big question is what you intend to do with this engine? If its to blow the doors off of ricers off the line then you want low end torque. To get this low end torque any 351 can be built to provide this but, if you still want decent idle quality, good vacuum etc. then you might want to consider a later model year (roller style cam). Some opinions out there claim you can get a more aggressive cam with this later style (roller) and not sacrifice idle and vacuum (which helps performance). However, if you want to run this thing continuously at 6500 rpm for example then block selection certainly does become important.
If the engine is meant for daily driving only then parts are available for all years.
Just my two cents. Currently building a .03 over 351W with comp cam, performer dual plane intake, 670 Holley carb, TFS heads and pistons and Hooker headers. I'm in the process of spending my two bits and finding out how she'll perform. Should be ready by mid June.
