The pedal boxes changed with the brake booster. So I believe the '70-'72 are all the same - the '73 is different, and then the '74-'76 are all the same. The pedal boxes had to change because of the spacing required for different brake boosters. The early clutch pedals are very different than the 260-280. You'll know if it's correct if the clutch pedal arm is thinner than the brake and curved along it's vertical axis.
If you're doing an automatic to manual conversion, don't change out the entire pedal box unless you have everything striped down to the bone. Just replace the pedals and associated hardware (springs, bushings, and etc.). The boxes were made to be interchangeable and accept the auto brake pedal or the clutch and brake for the manual transmission madels.
$350 sounds a little steep to me also, but if it's been meticulously restored with new hardware, and foot pads, maybe. The state of the other hardware might be just as important as the pedals. There are a few "wear items" that you would want to be in good condition. I have a few pedal boxes, maybe I should put them up for extra cash - naw, then I'd have someone come in wanting a swap...