The Little Engine That Could

The biggest difference between them is going to be in the heads...273 heads will require some careful work and prep to flow well...I've read a few places that have worked over the 80s 302 castings to do just that.

Hot Rod magazine did a build on a 318 with those heads and it made over 350fwhp N/A (yeah, yeah, I know...it's a magazine and their dyno is probably tweaked to sell copies, I get it). That being said, they did a lot of work to the heads and flowed over 200cfm @ .500 lift with 2.02/1.60 valves. I know you can't make that fit in a 273, but even with the smaller 1.78/1.60 valves or upgrade to 1.88 intakes, they'd probably do better than the older 920 castings and come with hardened seats from the factory. Did I mention that every 318 starting in '84 used them until the 5.2's came out? Here's the article:

http://www.hotrod.com/howto/113_0304_318_small_block_build/viewall.html

Now...that brings us to pistons...I couldn't readily find any forgings that would take that much abuse. I'm without doubt someone like Diamond could make whatever you want, but custom = $$...

...damn...maybe the 273 could turn into a 318/360?