The Buick Master Parts book confirms the change to side-mounted motor mounts began at the start of the 1948 model year at engine no 4999881. It also shows the 248 crankshaft introduced in 1941 was continued on with the same part number thru the last 248 in 1950.
The changeover to full-precision insert main bearings occurred mid-year 1947 after a trial run of some 1200 engines earlier that year. Another mid-year change occurred in 1946 to introduce semi-precision main bearings. So if it's a 1946-47 engine it could have one of three types of main bearings. Attached is a 1947 dealer service bulletin giving the engine number break points and markings for the various bearings.
What has been, can be again. (Bob Wills, 1942)