As
40 Delux mentioned, like all Y-block engines they can develop clogged oil passages to the rocker arms if they weren't maintained well... non-detergent oil and infrequent oil changes.
The 312's can have a tendency to leak oil from the rear main a bit more often than the 239 thru 292.
They do perform well in stock form, but as
Charlie said they are heavy, esp. when compared to newer engines.
Horsepower chart:
http://www.portholeauthority.com/thunderbirdETC/tech/engine.html
Should we assume you're looking at one to buy?
Is the engine from a 'full sized' car with T-Bird labeled valve covers, or actually from a '55 / '57 Thunderbird (there are several noticeable differences) or is it even a 312???
Look for the Letter Prefix of the (6015) casting number on the block. photo 1, see this link...
http://ford-y-block.com/Block%20identification.htm
Is it really a 312?
Some sellers can be 'confused' about what they have...
photo 2, only 312's have this dot at the rear of the crankshaft.
http://ford-y-block.com/identify312.htm
What are you hoping to do with it?
Depending on how complete it is, what it's actually from and what you'd like to put it in, the number & size of challenges will vary.
After all these years it could easily need a rebuild. That cost may be more than you expect.
If you can attach photos it could be helpful.