If you put big tires on it, or a traction device, the 340 might start breaking the 7260 U-joints.
But with 245s or less, yur good. Hopefully you have at least an 8.25 rear end with hopefully at least 3.23 gears
As for those 727s; either will fit just by changing the output shaft and tail, to the P-car length; no big deal.
Also no big deal is to upgrade the 904 for use with the 340. The 1973 model 340 is (or was), a low-compression iteration so its torque output is gonna be easier on everything behind it. But the 340 cam likes at least 3.23s and a higher than 318, stall.
Having said that, there is nothing to stop you from swapping the 340 cam out, in favor of something more streetable, to work with the 318 stall and gears. I'll tell ya; a 340 with a 318 cam is a pretty stout performer; it cures the soft bottom end of the 318, makes plenty of midrange, and gets great gas-mileage. It just lacks the 340 top-end rush, which with 2.76s you would never get to enjoy anyways until over 80/closer to 90mph, in second gear/ Jus saying.
Yes I already built such a combo, and now,45 years later it still remains one of my favorite all-time street combos.
Oh yeah, the 318 cam has ZERO idle lope, so your friends will never believe you when you say you have a 340..... until you fly by them in second gear.
You will need a "posi".