It,s time,6 out V8 in!Help with conv. kits!

The Schumacher kit is probably going to be the best bet as far as engine mounts are concerned. Going the NAPA route with the 273 mounts will yield a driver's side engine mount that won't fit. There are some who have gone this route anyway and shimmed the mount with washers to make it fit. I have not heard of any problems.
You'll need to get a V-8 steering centerlink. The /6 is straight where the V-8 one has a dropped center.
I have not seen early A-body headers that did not exit through the fenderwells. PAW or Schumacher may have something that will give you a break. The stock 273 V-8 driver's side manifold will be too small for the ports on the 360 heads. Suggest a later A-body 340 or 360 manifold.
If you have to put the engine in from the top, leave the 90º adapter or the oil filter off until after the engine is installed. Leaving it on will usually get the paint on the inner fender scratched. A Fram PH8A size filter will not fit without using the adapter. I have not tried the ½ quart version (PH16, I think).
The 273 4bbl used a single plane intake. I can't recall if my air filter housing rubbed into the hood pad or not. Suggest that you insure the intake/carburetor/air filter package will not require a hole in the hood, or worse yet, put a dimple in the hood where it impacts.
You may be tempted to run dual exhaust where the passenger side exhaust pipe runs under the cross-member. Everyone I have seen take this approach has at some point in time flatened the pipe. The successful ones have run both pipes through the notch on the driver's side. I'd be tempted to run the factory manifolds and merge them into a 4 inch single pipe with a Dynaflow diesel muffler (12770 ) on it and put an end on it to look like a 273 4bbl car. No sense scaring off good money before they see what it will do. Have fun!