SBC....

Changing rod length does nothing for compression.....how ever it does manipulate piston speeds around TDC...

A shorter rod has a faster piston speed around TDC and needs a larger head to take care off breathing at a higher rpm...

While as installing a longer rod slows down the piston speed around TDC thus absorbing more of the combustion power.... A longer rod also relates to less piston and cylinder wear over time.... for stock or mild street I would leave the stock size rods in it....but thats me....you could gain more by going lighter rotating assemble in a light vehical....

there are a lot of variables to consider when building an engine...

cfm is figured like cu.in X rpm / 3456

ex. 283 x 6500 / 3456 = 532cfm bump it up
ex. 283 x 8500 / 3456 = 696cfm