You do NOT know that it is a '73. Only the casting date or partial VIN will tell you that.

5c are 1975 only heads.

Year Engine Hp Code Valve Misc
1975 400 ALL 5c,6s 2.11/1.66 7.6cr
1975 350 ALL 5c 2.11/1.66 7.6cr
1975 400 ALL 5cx 2.11/1.66 7.6cr


Again, that does not positively mean your engine is a '75, only that the heads are. However, in 1975 there was a 400 YT with 5c heads. sc
1975 400 185HP YT A 7.6 066 5C 1-4 7045266 Block Casting # 481988/500557


If you are buying spark plugs (which fit the heads) buy for 1975 Pontiac 400. Those should be tapered seat.