64 Galaxie engine swap
I would recommend going with the 302, for several reasons. The 302 is the same block as the 289, so external dimensions are the same. 302 are a lot more plentiful, so you wont pay as much for a 302 as you would for a 289.
You didnt state what kind of transmission you have in your Galaxie. Knowing this will aid in the engine swap, as the FE block and SBF do not share common bellhousings or engine mounts.
On the 302, get a 66 or later engine with the 6 bolt bellhousing, rather than the 65 and earlier which have a 5 bolt bellhousing, making some swaping stuff a little more difficult.
If your car has an FMX type transmission, that is an automatic with an aluminum bellhousing and tailshaft and steel center section, all that will be require for swapping in a 302 to that transmission, is to go to the junkyard and get a small block bell housing. Since the 64's did come out with the small block, all you will need are the small block engine mounts to bolt in your 302. You might also consider swapping in another FE block, and save yourself a lot of parts hunting. The 390 was made from 61 through 74, and were found in everything from cars to trucks.
They all use the same bellhousings, motor mounts, etc. Another option would be to drop in a 360 FE, which was used only in pickup trucks, from about 68-74, but would also be a bolt in job to replace the 390, as the 360 and 390 are the same block. The 360 never gained much popularity so if you find one in good running condition, should be pretty reasonably priced. 360's in a proper state of tune werent too bad on gas, either. My Step Dad had one in a 74 F-250 with 4WD and after I would tune it for him, it would get 15-16 mpg on the hiway. Not bad for a truck with deep gears.
From your posting, I am presuming this is not going to be a permanent swap, but just an engine to use til you get the 390 refurbished. The 360, could save you a lot of money in the long run, as everything on your 390 will bolt right to the 360, including the ignition system, intake, exhaust, water pump, oil pan, and the accessories.
If you want more info on this dont hesitate to send me a PM on it.