If you do decide to rebuild it, be sure if the block is over-bored
0.030 inch, that the pistons you buy are also 0.030-inch over--and just as
important, be sure both dimensions are measured using *English inches*!!!
Seriously, IMO, it hurts NOT the value of the car if you use a 350 to
replace the tired 305. Both run regular fuel, both use a catalytic
convertor, neither is going to set records being stingy on gas, and the 350
has more meat in the water-jacket area for freeze-resistance and for maybe
yielding greater overall strength.
It seems to me that 1977 models weren't known for surprisingly great
fuel mileage; however, 11-12 mpg seems low for either 305 or 350. I'd be
checking the timing elements for being out of spec. and causing such poor
mpg. Both distributor timing as well as timing chain stretch can result in
timing being off. As the chain wears, the timing becomes retarded and over
time attempts have quite possibly been made to compensate by advancing the
And as has been suggested earlier, 70-K miles seems immature for the 305
needing a total rebuild. You did not say how much oil it was burning--but
you did mention the possibility of its being due to (valve) seals. I think
this is a good guess, and quite possibly new valve seals with a total
*cleansing* of the engine's interior, verification of proper cat. convertor
action, proper distributor function(including its advance characteristics),
checking the *slop* in the timing chain's operation to determine its need
for replacement, and a thorough tune-up(including new quality wires, dist.
cap, rotor, pcv-valve, plugs, & carb-service)--all this may please you with
how much better it operates.
You might be wise, though, to not expect even close to 30 mpg--either
US-gal. or Imperial-gal! I have seen some quite unreal vehicles that
yielded phenomenal & unexplainably good mpg(like the one S/R described), but
they were definitely among the minority. Just like I've also seen vehicles
perform on the other end of the economy spectrum: one I remember quite well
as being the most disappointing was a low-mileage 1973 Chev half-ton, 6-cyl
w/3-speed on column--no ps, no ac, no pb--truly a plain jane: it would NEVER
yield over 8 mpg, driving around my rural town or driving 55 mph along a
2-lane, low traffic road, to and back from a 30-mile-away larger town.
Started, ran, and performed as sweet as a 6-cyl could be expected. No
leaks, no cat. conv., of course, rolled freely, adequate power, poor mpg.
It was so bad, I refused to retail it in my small home-town. Wholesaled
it to a dealer and even told him it was rough on gas. He called me when he
got home to tell me he had to stop 3 times and put gas in it--apparently not
filling it, as dealers tend to do!. (Other 2 he bought did ok, I guess.)
And he lived only like 40 miles away. This was in 1975-1976 during tight
gas supplies and recent price-hikes.
Luck to you. s