340 manifolds will fit, although it's close. I have TTI headers now, but ran 340 manifolds for years (I've owned my 65 since 1968; it's been a 340 since 1973).
The late (71-up) passenger side, center-outlet manifold clears well. The early (68-70) hi-po passenger side manifold will barely fit if you clearance the lower inner fender where it folds around the upper control arm, and it may not clear some engine compartments give the the sloppy dimensional standards of the day.
The driver's side 340 manifold might hit the steering column coupler. It did on mine (with manual steering). I swapped in a power steering column, which is shorter, then used a manual/power adapter to connect the manual gearbox to the PS column. This moved the coupler back a couple of inches to where it cleared the exhaust manifold.
There are other possibilities, including late magnum manifolds and, of course, fenderwell headers.
Fenderwell headers are actually the easiest to install, if you don't mind cutting your inner fenders, and can deal with potential tire clearance problems, especially on sharp turns. I ran them for a couple of years (yes, the original inner fenders have been replaced).
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