I don't know anything about using Pascal, but it's not too hard to support System 6: Just use the raw MacTCP interface (MacTCP.p, if that's what that is) and use calls that are documented in Inside Macintosh Volumes 1-5.

Alternately, don't do any gestalt checking and code your code. See where it crashes on Sys6 to identify calls that require System 7. ;-)