fwiw, my old combo was 3.55's, an A500, and a 360 that was a reringed shortblock with home ported 360 heads, XE262 comp, RPM air gap, headers, eddie 600. combo would keep up with a stock 5.7 LX car, and I averaged 15mpg in mixed driving (19mpg best on an all highway run) on 87 swill (8:1 compression)....
I sold that motor, took my stock '86 short block, reringed it, had some junkyard mag heads redone with the suggested springs/retatainers, home ported them, had bullet regrind the cam with their HR259/316 lobes. used the same intake, carb, and headers. (9:1 calculated compression, runs fine on87 swill still)
power is pretty much the same as the 360, and I'm averaging 17.5 mpg in mixed driving, best tank was 19, have yet to do an all-highway run.
btw, a 273's bore is 3.625"....a magnum head's valves are 1.92/1.625"....that adds up to .080" under the 273's bore...plus add in the space between the valve faces...not enough room on a 273's bore....