While you can probably get a 300 to fit, it is not a direct swap for the 250. The blocks have nothing in common other than the # of cylinders; the 300 is actually related to the 240 I6 motor. Check out thefordsix forums
for more information. And yes, there are several examples of squeezing a 300 into the passenger cars.
On a side note, are you getting rid of your 250? I actually signed up to the board because I'm looking for the motor mounts from a 250. Let me know if you have them for sale.
My '69 Falcon is based off a shortened version of the '69 Fairlane. The Fairlane had the 250, the Falcon a 200; I want to upgrade when I need a rebuild. (Coming soon, block is leaking like a sieve, tranny is slipping, etc.) The 250 has a major advantage over the 200: it has the SBF V8 bolt pattern for the bellhousing, which means tranny selection is vastly improved. But the 250 is taller and about 3/4" wider than the 200; and it uses a longer snout water pump which gives clearance issues with the radiator/fan. I already have the frame brackets from a '69 Fairlane 250, and they are shorter and set-back relative to my 200 frame brackets. I'm just looking for the missing pieces of the puzzle.