65 Valiant swap 273 to a 360

That would be cool but unfortunately you are correct the converter will not work. The early 904's had an 18 spline input shaft and the 67 or so ups had a 27 spline input shaft plus a larger torque converter snout diameter. Your push button also would have a zero balance converter and you will need a balanced one for your 360.
Still, you should be able to find a reputable shop to modify snout and balance your push button trans converter to mate up to a 360.
Of course it will cost $
Pat Blais transmission is a good source for early to late modification
206-365-1966

And, 2nd issue, since I didn't read the thread all the way through, is that your 273 torque convertor WILL work with the 360. Agreed, the 273 is neutral balance and the 360 is not, all you need to do is buy a 360 spec balance torque convertor plate from Summit, or anywhere else.

Mike (still wishing for OR) in FL