The main difference between the engines was the heads. The LM1 used the 3932441 heads that had a 76 cc chamber yielding a compression ratio of 9.0:1, while the L48 used the 3947041 heads that had the 63 cc chamber. The smaller chamber of the L48 caused an increase in the compression ratio to 10.25:1 and thus increased the power output. Both heads were produced with 1.94" valves. Externally the heads had different casting features on the front of the head. A picture of the front of the head might enable a quick determination of the head type without removal of the valve cover to reveal the casting number.https://www.crankshaftcoalition.com/...identification
The LM1 and L48 short blocks were the same. Both used the same pistons, rods, cast cranks, and camshafts. LM1's have been found with both 2-bolt and 4-bolt blocks, though it appears that most were 2-bolt blocks. Most L48's appear to have used 4-bolt blocks, but a few L48 engines been observed with 2-bolt blocks.
There were minor differences in the distributor calibration, so the two engines used different distributors.
Both engines used the quadrajet carb. Both used the same intake manifold.
Supply the numbers on the front pad of the engine behind the alternator. If the pad is dirty, try to clean the surface without resorting to abrasives. Typically, you don't want to scratch up the surface since the surface itself has broach marks that add to authenticity. The numbers on the pad will tell us when and where the engine was built, what engine it is, and have a partial vin code that should match the last numbers of your full vin code on the dash.
If your build sheet matches the car, most likely the car has an LM1 if it's still the original engine.. The PO may have installed L48 decals on the air filter lid.