Only the SS 350 was 300hp. The LM1 350 4bbl was 255hp and was the replacement for the '68 327/275hp. They quit making the LM1 engine mid '69 year and replace it with the L65 2bbl 350/250hp.

 

Ted the LM1 was the replacement for the 327 in 69 the LM1 came with 4 barrel Roch. it was rated 255HP, it had dual fuel lines and it was stamped on the engine pad depending on which tranny. It is the easiest car to clone into an SS but the LM1 came in coupes or convertibles, RS or standard. It was discontinued around Dec. of 69.
LM1 engines come in 3932386 or 3932388 blocks as well. Try the Search function here in Engine section and also up in TAG section and type in LM1 there is alot of chatter about in the past too.
I have one

 

The easiest way on a complete car is from the engine assembly stamp. The heads are about the only thing different between the L48 and LM1.

On a car without the engine, the absence of front disc brakes would be an indicator although some LM1's had disc brakes. Some LM1 equipped cars had 10 bolts, but the same could be said about L48/Glide cars. An LM1 wouldn't have dual exhaust as a default, but could have had it added for an extra cost.

I suppose the easiest way (without the engine) if a person could prove that the car that they were looking at was not an SS, would be the presence of dual fuel lines which would indicate that the car originally had a quadrajet, meaning an LM1.

 

L48's (L34's and L35's also) also had dual fuel lines. The LM1 was only available before December 31, 1969 (roughly) so if it's made after that it won't be an LM1. After 1/1/69 the L48, L34, and L35 were the only cars with dual fuel lines

LM1 did not require the front discs, or dual exhaust that the SS's required, but they could have them. LM1's also don't require a 12 bolt, but could have one if ordered with posi. An LM1 would also have 14x6" wheels, not the 14" by 7"s that the SS's had, so if the tire pressure decal is left you can check that.

A 4 speed LM1 would use a Saginaw transmission and an L48 would use a Muncie so the speedometer hole in the firewall would be in a different location.

LM1's would not have the holes in the fenders for the SS emblems, or an SS emblem on the wheel shroud, tail panel or grille unless someone added them later.

LM-1's will have a silver grille, SS's have a black grille (unless it's an RS). If the grille is original the plastic will actually be molded in either silver or black as appropriate so even if the grille has been painted you can still check it out. Repro grilles are all molded in black so if the grilles been replaced you can't check this.

Other than these minor differences and not having an original engine code on the block theres not a whole lot to look for.

 

Originally posted by Mark C:
LM1's also don't require a 12 bolt, but could have one if ordered with posi.
A 4 speed LM1 would use a Saginaw transmission and an L48 would use a Muncie so the speedometer hole in the firewall would be in a different location.

SS350 and LM1 are very difficult to tell apart if the engine is MIA.

A few minor corrections:
- Most LM1's had 12 bolts. Posi had no affect on 10 bolt vs 12 bolt usage in the LM1.
- All LM1 4 speeds had Muncies. That's why they are an improved version of the L30/M20.

Sometime, I need to finish the LM1 article. And the seatbelt article. And the .....

 

All LM1 4 speeds had Muncies.

Wow I didn't know that! Thanks Kurt!

 

I can testify to the Muncie. I also have the 12 bolt with posi and front discs. Change the badging and the heads and SS here we come.