Transmissions through 56 (except the T85s that went behind the high horsepower Y blocks) will bolt in. Last fall I helped install a 55-56 top loader in a 52 sedan and it just bolted in. My memory is that through 59 or so is all the same bolt up locations, just the input and output spline count is different. Worst case scenario, you get a later transmission and have to swap clutch disks and drive shaft yoke to bolt it up.
Where are you located?