so i'm rebuilding my original 1970 340

Ok so the GVod and 3.55s puts your rpm right where mine is; 65=2240@ zero-slip. I can't get any decent fuel economy out of my combo at that rpm, with my 230@.050 cam, even with 10.95-Scr and 180psi cylinder pressure; the intake is just too dirty.
So that takes you back to the 223* SFT cam. And in steady-state running, and that 223 cam peaks at around 5000rpm, you don't need lightweight components.
If you want a lil more bottom end torque, you can even go down one cam size, it kindof depends where your final compression ratio come in at, with the least amount of machining.
Also, now, with what you said in post 14, not caring about hi-rpm power, then there is no need to spend your money on hi-rpm lightweight components.
But
I think RRR read you right from the beginning; you can slam a 273 crank and rods in there with some lightweight, tall, flat tops;
and put a 318 top end on it, OOps Rusty didn't say that part, lol. But you could, cuz they're good to 4000 or so,lol.( I already did that back in the 70s). But any old cast iron head will meet your needs, as long as you take care to not build a detonation-prone pos engine. Of course, a closed chamber, tight-Q, head will very likely get better mileage, partly cuz you can run a lil more pressure. But realistically, you already have the big-port heads so.......