The LS1 engine has better heads and lighter overall weight(90-100lbs). On any engine heads and intake manifold design is what makes power
That is why a LS1 with bolt ons and only a cam with stock heads can put more power than a LT1 with a 383 stroke.. it all comes down to the efficiency of the engine.
If you are going to buy a motor just get a LS1 engine.... It can make up to 400HP (350rwhp) with just good intake, headers and exhaust.
To make 350rwhp with a LT1 u will need to get an aggressive heads/cam or a SC running at least 6-7psi with all the bolt ons.
Dont get me wrong.... LT1 can make power, but it will take more money to make the same amount of power that LS1 can push.
There are few LT1's in the 1000HP range, but it took alot of money and time to get there. Hell give me money and i can give u a 700HP Honda LOL.
My advice is to go with the LS1 and just cam it and call it a day.
I gained 100rwhp over stock with just intake/headers/exhaust and cam. As you can see it doesnt take that much to make power out of the LS1.
Good luck