I went this route for my 66        Classic Performance Products     might have it for later models  

RACEY !!!!!!
6sal6

Also supposed to keep the shock towers from flexing (pivoting at the frame rail along with the inner fenders.  As both sides flex at the frame rails, they do so as a parallelogram, with both sides pivoting in parallel and the top of the parallelogram pivoting with hinge points at the connection to the inner fenders, parallel to the bottom of the parallelogram.  In that scenario, the heim joints do nothing to stiffen the pivot at the monte carlo bar to inner fender connection. They maintain the distance between the shock tower tops, but do nothing to keep the parallelogram from flexing from the rigid rectangle it is supposed to be.  Having a flanged, not pivoting, multi-bolted connection at the end of the bar helps maintain the angle in a stiffer state, keeping the shock towers vertical, minimizing the flex at the hinge point along the top of the frame rail. 

Stiffened by alot?  Probably.   Enough you will notice on the car you drive on the street?   Who knows.     My point is that you need to be looking at the frame SYSTEM of all the parts involved in the equation, not just some barious method of attaching the ends.

This reminds me of the scenario where the restaurant manager tells the bus boy to go wipe down the table.  So, kid wipes the table.  What the customer wants is more of the big picture, with the result being a clean table, not just a wiped-at table. 
The purpose of the monte bar is to stiffen the whole front end of the car, not just to add a bar securely to the car.