I just recently put a '73 400 in my '69 that originally had a 350. I had no problems.
The blocks are exactly the same physical size, so there are no "fitment" issues.
The only thing you need to check is your motor mounts. The best way to check this is to measure the distance between the bolts on your existing motor mounts on your 350 block, and then the distance from the front of the block to the first motor mount bolt. Compare that to your new block to make sure it is a bolt-in fit.
Most mid-70's blocks (like the 73) were made to fit both the older 60's mounting locations, and the newer 70's mounting locations. You should be ok, but please double check.
