Gentlemen
I recently purchased a rather nice 1987 420 sel PARTS CAR
it has documentation on the timing chain / head /seals and other maintenance performed , to the tune of $2700 in Feb of this year. I picked the car up in State College Pa and drove it home to Baltimore 135miles - No issues , really a nice car .

( seems a shame to part it out- but the price- low enough and I need some items for my 89 300 se)

A couple of questions;
1. The differential ratio on my 89 300se is the 3.46 the 420 sel has the 2.47
assuming they are interchangeable -
WOULD IT BE FEASIBLE TO SWITCH THE DIFFERENTIAL?
Might I realize better fuel economy at highway speeds?
or would the 6 cyl not perform properly ( not certain of optimum rpm range )
would my start up be horrendously slow /tedious ? ( 240 D like ?)
Currently I realize 19/20 mpg highway cruising and was curious if the 2.47 ratio might get me to 24 +- mpg at 70 mph cruising speed?

2. Is this 420sel drive train( 4.2 l v-8) suitable for a transplant ?
such as the '73 280 in EVOLDEAL'S THREAD ?
http://www.peachparts.com/shopforum/showthread.php?t=289579

or a locally owned '72 300 sel ? in need of new engine/ transmission

Thanks in advance for any input.