1967 Barracuda - Slant Six (orig) or 440 Big Block? Daily Driver, Need MPG

So I'm in the works of restoring my 1967 Barracuda Fastback and I'm going to be putting an engine in soon. I have two options.

The car was originally had a slant six inside of it, but the guy I bought it from was in the process of converting it to a 440. I'm not sure what he all changed, but I believe the original transmission is still inside of it. He sold the /6 and rebuilt a 440. The 440 ('74) is ready to go, complete with radiator, fans, starter. But no transmission. I picked up a slant six for 100 bucks, and the guy gave me a new starter, alternator for it. But it needs a rebuild and overboring.
I'm transferring universities so I'm moving to a bigger city (SLC) and I need the car for a daily driver.
What are the average MPG of each?

440
-Rebuilt, ready to go
-Has Radiator
-No transmission (my friend might have one I can use for free)
-Also he knows a guy that can put both in and add some reinforcing for it for a couple of hundred, I just need to buy the motor mounts (it might have them on already)

Slant 6
-Needs rebuild
-Has Radiator
-Has Transmission