I'd buy the repop red spring listed in the year-one link (or better yet, someplace that doesn't hang you for shipping) and be done with it.
Yea... I know... It's just a light spring that holds the trans linkage against the carb linkage. Just about any light spring would work OK. My thoughts are the spring tension was chosen to allow the throttle to close and the trans linkage to stay open or return slower.
IMHO, since the linkage controls the line pressure that operates the other functions of the transmission, it's good practice to keep everything functioning the way it was intended. Small details like adjustments make or break a good running car and while this spring tension may not be real critical, it is part of the operation. It's a cheap $10 and if nothing else, it looks "correct".