I would look in a junkyard you can get a 351w easy ford used them tell 96 in the trucks.As far as what both are worth depends where you live around here most pull the mod motors out of the mustangs for a small block.

 

I'd probably trade if I were you, providing the 351 long block is in good shape. You will have to get a new intake manifold and adapters for the accessory brackets to account for the taller deck height. I think you're also going to have to get headers that are designed for the swap.

 

Look around online and see what 4.6s and 351s are going for to get an idea of relative value. My guess is that you'll do fine if you trade the 4.6 that you can't use for a 351 that you can use in your 68. They you'd have the 302 to sell to make a little profit.