Honestly...how good was the 318...mpg / durability / hp

Just BTW- all the 318's I've ever had have outlasted the cars and/or transmissions they were attached to.

I have also found that both 318 and 360 engines seem not to have consistent power output.
Any given one could be a screamer or a puppy.
This seems to have carried over to the 3.8 as well, as I have observed some pretty dramatic differences in power on that particular engine.

Another thing that seems to be inconsistent on the 318, is oil pressure.
I've had stock, unrebuilt 318's that have:

Carried 20# at idle and only gone up to 50# at 4k RPM
Carried 15# and gone to 90 at 4K
Carried 78# and only inched to 80 at 4K!
...and almost anything in between.
All engines ran fine.