I suppose it's late to jump in now, but considering how often we are advised to drop the later engine in our early 50's trucks, and how easy this swap is supposed to be, I would expect the 53 engine would bolt right in, easy as pie. The '53 engine is a low pressure babbit-pounding motor which has a number of disadvantages compared to the post-'54 engines, so many of the wiser guys would recommend that you try to find a slightly newer engine.
This question would have gotten a lot of quicker replies on the General Truck Talk Forum...
Have you bought it?
Karl

It may not be the easy way, but it is the Cowboy Way - Ranger Doug
Beware of the stories you read or tell; subtly, at night, beneath the waters of consciousness, they are altering your world. - Ben Okri
1953 Chevy 3100
1960 Volvo PV 544
1941 Chevrolet Special Deluxe