I'm not sure about 53-54 pontiac interchanging with 49 52, but I do know that 4 door seats are different than 2 door, and if your 50 is a fastback, the seat top is unique to the fastback series.
Were Working Panels For The 54 Chev Now And Then 49-52 Is Very Close If Not The Same In Many Area's Especially With The Seat Rise Area So I Belive It Will Be Ok, But Like Former Post The Body Style Is More Important Then Year