Need assistance with a shopping list to convert '68 GTS to 73-76 center link

How much (if any) clearance will I gain?
Honestly, not a lot. But sometimes that's all you need.
Picture your existing centerlink with the tie rod ends and pitman/idler arm studs pointing down instead of up- that's what you'll gain. Sometimes the studs pointing down will gain you a bit of oil pan clearance.
Just remember, clearance gained in one direction is also clearance lost in the opposite direction.