Pull out the motor to swap transmissions, is easier (but not necessarily) if you have to pull out the auto it's easier, if YOU DON'T need to pull out the transmission because there is not one there, you can install the manual, i'm not too knowledgeable on this topic, I'm sure someone else will chime in

 

so u mean i can just pull out the auto tranny and put a manual one one in with in with the clutch and all the other stuff needed for it with the same motor

 

You certainly can.
I converted an automatic Legend to manual with little difficulty. I left the motor in the car as I didn't want to unhook everything. If the car needs the timing belt, water pump and oil cooler hoses replaced then I would probably suggest pulling the motor out and giving it a good tune up, bolting up the new transmission and reinstalling.
Whatever the case, give it a go and be sure to get picures

 

Don't trash the auto tranny out of your '90 LS. If it's still in reasonable shape, it may be worth something to some folks on our forum. The auto tranny for 1990 changed from the 86-89 years and is harder to come by.

 

+1 If your gonna make your car to manual, post up your auto tranny up for sale here. There are a couple members that need it.

 

You could give me your auto. I could always use an extra

Mine's kinda rough and it's a bit noisy shifting when it's cold. I'm a little worried about it, although it's probably just the solenoids.

 

if you have access to a lift, don't take the motor out.