You can do a head gasket job for around $300 with all new engine gaskets and materials. You never know what you're going to get with another motor. Sticking with the one you got is your best bet. Unless you've overheated it too much, too many times.
If you want to swap in the 251, you might as well only buy the block. You HAVE to use the top end and all sensors from a 25D.
Now, even better than sticking with your motor entirely, do a block swap. Not only will the reliability increase, but power too. A 251 block has a MUCH better bottom end than the 25D. It does not have the bearing issues. Compression will increase to about 11:1 - 11.5:1 and you will find that you have much more power and low end torque.
Either way, the engine has to come apart.