If your mopar is you daily driver!
if money is tight, conisder one of the 833 overdrive trans, should be able to do it cheap. a good running small block with good dual exhaust, chould do pretty good. i jsut put a 6 speed and 400 in my duster i mocked by some local slant six guys, but i tried to live wit hthat slant after hopping it up, and just wasnty thrilled with it. so far i got 1 mpg better than i ever did with the slant and i havent had a ton of time to tune the carb. it wasnt exactly cheap, but i did what i had to do so i could drive the car long distances without going broke. you can get ok results from a slant with some ho up parts and tuning, but just about any late model car will kick your *** and get better mpg.