What engines can I swap out a 69’ Pontiac lemans with ? Preferably something Ls

The Lemans used the same basic size engines as the Malibu so the base V8 was a version of the Chevy 350 block. The body will also accept a similar version of the 400CID used in the GTO's.

The Pontiac 350 is NOT a Chevy 350. Same size, different engine.

I would keep the car original and rebuild the engine or at least use a correct Pontiac engine instead of a Chevy engine.

My mistake - old brain but I believed the chevy block was the same as the other GM engines of the period. Sorry and hope you find a suitable replacement.

Old Pontiac blocks are visually the same. Technically they never had a big block like Chevrolet instead the larger 428 and 455 used a larger crank journal 3 to 3½ but blocks exactly the same making swapping easy. If you want more hp for your buck crate motor is the way to go.

Correction on the journals 3-3¼ and 421,428 and 455