don't bother with a 273. pistons are too expensive to get high compression/0 deck. copy hotroddave's 318 build...

the key to mileage is high compression and little overlap. you'll get neither with a stock bottom end 273 and that cam.

or another option is his short block (KB167 pistons at 0 deck), magnum heads, lunati voodoo 60400. the magnum heads will make significantly more power than the 273 or '302 heads.

or use an '85 up 318 (roller cam motor, 302 heads or magnum heads) with the KB167's, and get the roller cam reground by bullet cams, probably with the HRL256A lobe on the intake and exhaust if using magnums, the HR259/316 on the exhaust if using the 302's...if you're maximizing MPG, use the HR247/306 lobe--similar adv. duration as a stock cam, but about 10 more .050 dur than a stock LA 318 cam, and a lot more lift (almost as much lobe lift as a stock cam's valve lift!) use that lobe for intake and exhaust and you'll have 23 degrees overlap on the stock 112 LSA

my combo is a stock '86 short block, pistons .055 in the hole, magnum heads home ported by me, magnum roller lifters, pushrods, and stock mag rockers, thin head gaskets (9:1 comp measured) RPM air gap, headers, eddie 600, and the roller reground by bullet with the HR259/316 lobe for both intake and exhaust.

on the butt dyno, engine has as much power as my old XE262 equipped 360, but I'm getting 3mpg better (18 vs. 15) in my 3900 lb 5th ave. the great thing with roller cams is the aggressive profiles...my cam has 10 degrees less .050 duration than the old XE262, but only 3 degrees less duration at .2", and more gross lobe lift. area under the lift curve is the same or better than the old cam, but with 13 degrees less overlap.