Glad to see the work progressing.
I have never heard the side mount thing started mid year. Maybe so. From my knowledge and what I see. Side mounts started in '48, your engine has none, therefore a '47 or earlier. Other than mounts, I believe no difference. Still a 248.
Insert rod bearings started in 1949.
First 263 was 1950.
All 248 had increasing size main bearings, front to back.
All 263 had same size main bearings, front to back.
Your rods can be machined to accept inserts. Relatively inexpensive, I
believe.
If you will post the engine #, the year can be confirmed. Besides being on the side of the engine just in front of the distributor , it is on the front beneath the water pump, in that bluish area on the front of the engine.
I really like this project.
Ben