If you want an actual GTO 400, engine (67-73) they are very hard to find .. the average run of the mill 400 Pontiac engines are even getting scarce in running condition depending on where you are the 400s were produced from 1967-1979 (actually the 79 engines were leftover 78s) and were available in the Trans Ams that year ..
the last 400 engine put in a GTO was the 73, the 455 was also offered until 76 in T/As
its production started in 1970..