dodgefreak
A 273/318/360 isn't going to idle any better without getting the right cam. Your just wasting time and money changing the 360. Oh sure, the 273 MIGHT--and its a maybe--get 1 to 3 mpg better--built the same.
do the math sometime, you're saying this but meanwhile we are talking an 87cid difference here..even built the same, theres gonna be a very noticable diff in milage.
Get her a Comp HE 252 and a 600---maybe even 500 cfm Edelbrock carb to top it off. Make sure the intake has a perfect seal--get rid of those leaky headers if it has them, get cast manifolds and she be tinkle pink happy...and it cost less and be done quicker.
