Your fine mounted in the manner you show, did mine to the same locations without issue, though my mounting was a bit different in the structure.

The real choice you face is limitation of mounting position;

As shown, you'll need to touch up paint if you leave it on the jig, but it's horribly convenient when mounting your pre-assembled rear axle/spring combo and checking fitment of the rear valance.

Mounting to the spring perch makes for better paint access but makes for a pain while trying to remove and reinstall rear axle/spring combo. This location also eliminates you from checking the fitment of the rear valance unless you intend to temporarily take it off the jig.

As for balancing, use the whirly jig instructions, the base concept is that as the car is raised relative to the rotation center point, a balance position will be realized. This location may change if you mount the car with anything on it (perhaps glass still in doors, front suspension components, ect, ect)

Working on the jig is a real saver, the benefits far out weigh the inconvenience of time to mount the car.

Whirly Jig recommends using round bar stock running the length of the system in each of the lower outboard positions, for several reasons I chose instead to use square tube steel sections with an oversized coupling in the center to join the lengths, this now gives me better flexibility to have correct length tubing runs based on the cars actual length with minimal excess tubing protruding from beyond the jig. This approach also makes it easier to transport should you decide to move the jig less a car.

What I do is rent mine for minimal money, gets free storage and helps pay for itself. If I need it in the future it's available for reclaim, though I admit I rent only to trustworthy friends.

Good luck whatever you decide for mounting position and setup.