talk me into an ls swap..

[Squareforceone]

well i just put all sorts of money into a used 350 only to find out it may have a cracked block i don't want to put more money in a bad gas mileage engine anywys... I have 3 criteria that i need to accomplish for me to decided to do an ls swap. 1. mpg i need to get about 20 is this possible? i have a 700r4/will get a new tranny and i think 3.08s 2. if i can do the swap for 3000 or less that's for everything. kind of low i know but i don't want to do anything to the motor just leave it stock and probably not even run ac for now. 3. get it done by the end of summer. Is this do able? How much money/time did your swap take and what mpg are you averaging. I'd be selling my daily driver to fund this and so i need the my truck to get good mpg to make it worth it.