Re: 351c or 302c?
The blocks are physically the same. The crank casting is correct, if it says 302 that is what it is. The 351 has a 1/2" longer stroke and 5.7" rods to compensate. It has 351 on crank, maybe 4MA, 4MAB, or some other jiberish - these are typically American origin cranks.
The heads are different as well, closed chamber 2v for 302 and open chamber for 351 so they both ended up with around 9:1 compression.4v heads are around as well(can nearly put fist in intake ports)
Most people do not build up a 302c as the extra cubes of a 351c is hard to pass up, all of the extra torque. All you will need is a 351 crank, you can buy 351 pistons to suit the longer rod and you should have a set of closed chamber heads in your possession.
351c's will give you 300hp pathetically easy with the sky the limit for increases, money, money money.
What is it going in?