First, if I was in your shoes, YES, I would do the swap - I think you will get that idea from most of us here. The good thing for you is that you can still drive the car and enjoy it while you source the parts. There are a good number of fellas on this site that can either provide you with good parts for a good price or steer you in the right direction... especially for a 1957 Ford. '57 was Ford's best performance year for the Y block as well.
I recommend that you do the following:
1. join a car club or find some folks in your area that own your make and model... would be good to locate some Y block people. Where are you located?
2. Purchase two quality books - look on ebay and purchase the Y Block book by James Eickman. It is not perfect but it is an excellent resource. The other book just simply go to Amazon. It is a BRAND NEW (and I mean most of us just got a copy last week) release by an author named Charlie Morris. In the book he gives a stock rebuild and then a performance build at the end. Loads of pictures and great information. Search "y block book" on this website to get the details and the reviews. There is a thread in this technical section about the book itself.
3. Stay plugged in to this website - you will not find a better group of friendly, knowledgeable people here that will help you with your Y block swap. They have been invaluable to me, that is for sure. Browse the previous threads to get info.
4. Keep this in mind... You need to know up front that the 312 was Ford's best y block out of the plant, and it is very desirable. It may be extremely difficult to find a good block that someone is willing to part with. 292's are vey plentiful. You may want to do what a lot of us have done - find a 312 crank and have the mains turned down to 292 size so that you can essentially have a 312 in a 292 block. Most all 292 blocks will bore out to standard 3.8 size of the 312 block cylinders.... check with a machinist to be sure.
5. Make a spec list of what you want behind the 312... this will affect cam choice and so on. Do you have a manual or automatic now? What about that rear end? 57 was Ford's first year for the 9 inch... you may elect just to swap out only the engine and transmission.
6. Consider the route you want to go... I do not know if you want stock or hi-performance, but the difference in price for your build or parts acquisition can be quite a bit. Most generic rodders and car enthusiasts stay away from the Y block because parts are not cheap and some parts just are not available. Just until a few years ago, the Y block had NEVER had an aftermarket head made for it.
7. Subscribe to the Y block magazine... there is a link and info on this website if you look under links.
As for Why? around here we say "Y not a Y block?" for Performance, Reliability, and Nostalgia, you cannot beat a Y block, hands down. Look at the 56-57 NASCAR record and see who won the most races... Y BLOCK powered vehicles. Ford used the 292 in their trucks up until 1964. In South America, Y blocks were used up until the early 70's (1974 was the last year I believe). You also can't beat the appearance of a Y block, and for the record, at car shows it has become very common for people just to throw in a small block chevy or a small block ford and be done with it. Almost cookie-cutter like.
We are a different breed - that is for sure.
Lancaster, California
aka "The Hot Rod Reverend" 
check out the 1955 Ford Fairlane build at www.hotrodreverend.com