You can use a vacumm switch from a mid 80's GM to signal when to lock up the convertor. If your doing this for fuel economy you will pick up a little bit but maybe not as much as you think. The reason is that in OD the airflow is reduced and thus the venturi signal - you may find what was a crisp carb response in drive not to be as good in OD. After you do the swap let me know if this is the case...
The linkage out of a mid '70's nova with a TH350 should work, the 700r4 does need the linkage to be set properly otherwise the trans will cook from low line pressure. Installing a line pressure gage on the main pressure port above the trans thottle linkage is a good idea while setting things up - you'll need a 0 to 300 psi gage and hose good to 300 psi - check local hydraulic shops.
