I can barely spell LS3...no idea what a 408 means or is. Is it sold new? The LS3 is sold brand new as a package...That sounds appealing. Pretty sure the torque of the LS3 will be a MASSIVE upgrade from the OEM Motor.What about going with a 408 rather than a Ls3? The more cubes the more torque down low. Just a thought.
Whats the 4L80?I just swung this into my 62 yesterday. NV4500. They are very stout and although the gear spread is not as "nice" as an H55 an LS V8 doesn't care. Especially an LS3. I have no love for 4L60s. If you are doing a GM engine there are 2 proven options IMHO, NV4500 or 4L80.
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If it is the convenience of getting into a warm truck you're, look into getting a coolant heater and or oil pan/block heater. Heat right away and less wear on start up.
Take a look at one beside a 4L80. Take a look at one beside an A440. Pick one of each up into the back of a pickup. Take them apart and have a look inside...So, the Auto transmission that is sold with the LS3 isnt "proven"?