How To Get 351w To Fit Into '73?

I know special swap headers are required for the older ones. But the 351 was an option in 73 so you may be ok as far as clearance.
1) 351w block is slightly taller, so your current 302 manifolds/headers might not meet up with your exhaust underneath. If that is the case, you will need the stock 73 351 manifolds or headers made for the 351.
2) Since the deck hight is taller on the 351, the intake width is also wider. So if you were planning to, I dont thing you can resuse the 302 intake manifold. Most of the rest of the accessories should transfer though. Not sure if you were planning on resusing this stuff or if the new engine already has it.
3) You will need a new oil pan as the 351 pan is deeper to clear the larger main bearing. New pump as well.

any front sump 351w oil pan will work just fine. the rest is mostly bolt in stuff as the 351w uses the same mounts as the 302, as well as having the same bell housing bolt pattern. its the little details like throttle linkage and wiring, alone with accessory mounts that will cause a few minor headaches.

oz said:

I don't think the 351 Windsor was available in '71-'73 - only the Cleveland... Does the right oil pan and pump combo exist?

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In that case, if the Cleveland fits with no clearance issues, then the Windsor will have no problem. Any standard 351w pan and pump from any online mustang store should fit. Or if you get a donor, the front sump is what you need. Not sure what kind of hood clearance you have on a 73. I'm rebuilding a 351w to put in my 66 and the hood clearance with the taller deck is an issue. I had to be selective about my intake and air cleaner.