Well thanks for the input venom and daytripper. i'm not sure how but its been construed that I'm desperately after 20mpg, originally only asked if that were feasible. Its not, so moving forward.... I actually just want better than the 10-11 I'm getting now. 15+ avge would be a good result.

Venom if you could provide me links or info relevant to your LS conversions that would be much appreciated. i'm looking round auctions, sites and wreckers for donor vehicles and trying to narrow the field. Is a 5.3 efi, 4L60E and regular rear end gearing an appropriate combo, again I'm all at sea here and seeking wiser counsel. Thanks again