Is it possible to put 351 heads to a 302 block?
How can you know if it is a 351 or 302 engine, are there any marks?

 

yes it`s possible ... 351 uses bigger head bolts so it is usually recommended to use washers under the 302 bolts .. the 351 is about 1 inch and 1/2 wider than a 302 intake .. .. the only good heads for this swap are of the 1969/70 era .. even then you will lose some compression and power .. step up to some aftermarket heads if you need some power

 

What if I found someone selling a 351 completr engine, how can I know if it really is a 351?

 

unless your`e really hip on them it`s hard to tell the dif .. the 351 will be 2 or 3 inches wider across the intake and valve covers .. i will get you a measurement off mine when i get home tonite ..

 

I have put 351 windsor heads on a '68 289 many moons ago. I used shoulder bolts because the 351 heads take a larger diameter bolts than the 289 does. Also you need to drill a hole in the head to align the water jackets. Other than that, easy cheesy.
Let us know which direction you decided to go.

 

I have Roush Windsor junior heads on my 289 beast. You don't want just average everyday Windsor heads from some parts motor, they aren't worth the hassle.

One way to tell even with the heads off the block is that they should actually say 351w on the head under the valve cover. It is part of the casting.