The "best" is 25% on each wheel, with the mass as close to center as possible.
Weight behind the rear axle or in front of the front axle is bad!
Side to side is important also, and that includes passengers.
I weigh 250 lbs. so I need 250 lbs. of ballast (batteries) on the other side, for balance.

 

Unfortunately the batteries in the back will be behind the axle. I just found I can fit 3 (Tesla) modules in front, so now I will have only 100 lbs in the back, as I don't have to build a box for them.

There must be a thread on here someone knows about with a thorough discussion, no?

Jim

 

There are cars with 60/40 front/rear and rear/front - you could probably go up to 70/30 without any real issues,

The "keep the mass central" is to do with "polar moment" and it is a two edged sword,
Everything in the middle means your car will react fast to any force,
Which is good - unless you don't catch it then its bad

Basically as long as you change your spring rates and your brake balance to suite your new weight distribution you will be OK

 

If as you say the car will still be lighter than originally, then there should be no issues.

Just imagine what kind of a weight distribution a pickup truck has. Some are over 70/30. A lot of FWD cars have around 60/40 distribution while rear-engined cars such as Porsches have around 40/60 distribution and both can still handle great.

I doubt the average person would notice even a drastic change in the front/rear weight distribution of their car unless they frequently take it to a race track.

The main noticeable difference will be that after taking out the weight, the car will probably sit a bit higher and possibly ride bouncier. This can be mended by changing the length and stiffness of the springs and shock valving, depending on how much it will bother you and how much you want to invest in it. I am not sure how many off-the-shelf shock and spring options exist for a 1966 Volvo.

As Duncan pointed out, if you find the brake proportioning to be too out of balance, for example if the rear wheels are more prone to locking up when braking, you can add a brake proportioning valve to the rear brake line.