318 vs 340 Questions
I've had two 72 340s, both had forged cranks. I've also seen cast cranks in late production cars.
I've got a '71 build (early too, can't remember date) '72 340 with a cast crank and external balance. It's before the year's end of '71 for sure.
There was a minor change in HP for the '71 year with the switch to 2.02 J heads and IIRC the '71s got TQs. '72s had TQs, 1.88 J heads like the 360s and low CR pistons- most likely gonna see external balance on a stock motor. '73s got 587 heads with 1.88s and low CR pistons with external balance.
So then the final part- which to build. Ask yourself how sure you're gonna keep the car. If it's a given, doesn't matter- save the money and build a 318 or 360. If there's a pretty good chance you may let it go down the road- do the 340. A 340's worth a lot more to a car's value than the motor will sell for.
As far as fuel mileage is goes, if you are concerned with it buy a Prius. I built a bunch of small and big blocks over the years and never considered fuel mileage. My goal has always been to make them run hard for what they were. I guess I'm one of the dinosaurs that likes to build and drive fast cars so my perspective is skewed.
Lots of minor changes can really change a car's fuel mileage. Sure, it's easy to say you shouldn't be concerned about fuel mileage, but the same 12mpg+ difference you're referring to between a boring econobox and a RWD V8 can be lost in a build.
Sure, it ain't gonna get fuel mileage if it's really built to run no matter if it's a 3G, SB, or BB- but in between there's a lot of compromises that can make a difference. A 3spd auto with steep gears rarely will.
yabut he already owns the teener, and the difference from 390 to 408 is, for a street car , um, well IDK..........
But to support either he's looking at so many other things, it's like; where to start?
And that's why I think buying the cheapest, best motor is a sound idea. Which whenever I see is usually a Mag 318. The old teens never came with very good top end parts and that was their big downfall.