fuel economy with carb

[markallen]

right now my 72 has a 250 in it. i would like to rebuild it and fix it up, however i am 18 therefore i am terribly broke and it costs more to fix up an inline 6 than it does to buy a crate small block so im thinking i am going to either buy a crate small block or rebuild one of the 350s that we have and just save the 250 for a future project. jegs has a 290 horse 350 long block for 2000 with a 3 year 100,000 mile warranty but my dad has a 350 just sitting around that i can have to rebuild (this motor is running but has plug fowlers on it) my question is should i buy the crate motor or rebuild the other one i would like to have a little more than 290 hp but i would also like to have that warranty also once i get the 350 in what would be the best setup (transmission, rearend, etc.) for the best gas mileage without going EFI (because once again i am broke) i would like to have around 350 hp and still get atleast 15 mpg is that unreasonable?