If the shifter is about 11" back from the front of the case, it should be out of an S10. Most S10's used a 4.03 first gear, but many had around a 3.50 first, which would be the one to get if possible. I hope it has a mechanical speedo? it will cost about $300 for a gizmo that will allow you to use a electic sender with a mech. speedo.
I believe your gear options for a '57 car were 3:55, 3:70, 4:11. Maybe 3:36 or 3:08, but I think those came into use a little later. I would have to check my books to be sure.
When using an S10 T5, you will need to shorten the input shaft collar and trim a little off the tip of the input shaft. use your 3spd for reference. Since the T5 is all metric, you will also need to drill out the mounting holes to 1/2". You will need to change the clutch disc because the spline count on the T5 is different.
When you get your disc, test fit the T5 and disc without the pressure plate and make sure the disc can move back enough to disengage from the flywheel. If not, then some grinding of the hub splines will be needed. Once that's all done, you can reassemble everything. Throwout bearing, fork, pressure plate can all be reused from the '57.
The T5 is longer that the 3spd, and has a different tailshaft spline count, so a driveshaft will be needed. I don't know if it will work in your car, but in my '53 truck I used one from an Astro van and changed out the rear u-joint to work with my '57 car rearend. If you cant find one that will work as is, then you will need to get your made/modified. Probably looking at $75-$150 to do this.
You do not need to add a tranny mount. The stock 4 point setup is fine as is.
Any other changes to the floor, etc.. that might be needed should be obvious once you get started.
With a stock camshaft and 4.03 first gear and a 24-27" tall tire, I would say that a 3:36 would work quite well for a daily driver. You may find first gear to be mostly useless with a 3:70 or higher ratio, but its not the end of the world to skip first. Just get it all put together and see how you like it before you change the gearing. Remember to check your speed, as you will probably need to play with the speedo gearing.
