Classic/Antique Car Repair: fuel efficient c-10?, running gears, cam 350

Question
howdy

i've had a 64 chevy 1/2 ton pickup for about 5 yrs.  w/ recent
gas prices & a new job w/ a daily round trip of almost 100 miles,
i got tired of my 13 mpg freeway drivin'.  so i decided to
upgrade her a bit when the engine finally blew.  the previous
owner had put a 350 in the truck.  i don't know what the lift &
duration of the new cam is but i told the rebuilder to use the
most fuel efficient cam he could get.  then we put an edelbrock
500 cfm (down from 750) 4 barrel carb  & new fuel pump on.  
now it really gets out of control.  we swapped the 350th tranny
for a 700r4 (4 gears overdrive) & swapped the 3.7 running gears
for a more freeway friendly 3.08's.  i spent about $4k on all this
& i swear i'm gettin' 10 mpg now!  what the hell did i do wrong?  
i've got a hotrod 54 chevy that get's about that much w/ a .040
over 350 engine w/ 650 carb, hot cam, 350 tranny & 4.33 posi
rear end.  this doesn't make sense.  any help would be greatly
appreciated.  also, the engine rebuilder said i'd need to have the
valves adjusted after 500 miles.  i haven't done that yet.  could
that really be the solution to all my problems besides replacing
all the quarter panels w/ fiberglass?  ugh.

thanks,
dave

Answer
Dave
  I wonder what type of cam that he used ? and what is the mileage on it , is it new , as it might have to break in. I would think it should be getting better than 10 miles. and as far as the valves adj. burning fuel ,it should not but i would ask what cam is in it.also the fuel now a days is low oct. and that to can cause a waste of gas in a v-8

 ok   Roger