OK my orig motor is all iron. Intake, cast manifolds, water pump, ect. LS has the following alum- heads, block, oil pan, valve covers, w/p, acc brackets, plastic intake, you get my drift. Im also adding headers, losing the a/c compressor & mechanical fan, going w/ a smaller p/s pump (ls style is very small).
Now if you already have added things like an aluminum intake, headers, w/p, things of that nature, of course you arent going to get the weight difference. LS is 380lbs fully dressed. A cast 350 is roughly 550lbs bare (google search says that anyway) Add in the cast extras plus the weight I lost going to an aluminum bellhousing, flywheel, and transmission and the numbers come together nicely. If you were to just swap to the LS already having everything aluminum in your 1st gen 350 you can still pick up 200lbs.