How is the battery secured in a 1987 328?

If it’s like my 85 308, there’s a plastic block with I believe a 10 mm bolt that secures the battery to the battery tray.

 

"The hold-down clamp is located in the front center. Using a 13mm deep socket on a long extension, loosen and remove the clamp plate, nut, and lockwasher:"

 

A typical engineering term would be a "toe clamp" (that engages a step/ledge in the bottom edge of the battery case) -- item 6 here:

https://www.ricambiamerica.com/car-diagrams/ferrari/v6-v8/328-group/328-1985/electric-generating-system.html

 

clamp not secure battery good i use strap around tray to hold battery better for hard cornering too

 

If the correct hold down is used and the correct battery is used you could turn the car upside down and shake it and the battery would not move.

 

Yes, there should be a male threaded stud protruding upward from the battery tray frame in front of where the battery sits. Any metal nut (of the correct thread and reasonable thickness) and washer should be OKish. It is important that the battery used be fairly close to the stock battery size (i.e., if someone previously used a very narrow battery size, the toe clamp wouldn't reach). Are you confirming that the toe clamp piece, item 6, is just completely missing?

 

The first time I changed the battery in my ('88) 328, I wasn't all that confident, so I took it out for a spirited set of corners to verify it was secure. ("Angles and dangles", in submariner parlance. )
Since the battery case is one molded piece, clamping the base down solidly will hold the battery secure.

I recently got a selection of "C"-clips off Amazon, so I could replace the missing c-clip on one battery cover knob.

One other hint: putting a magnet on your socket and extender will help assure the nut doesn't drop into inaccessible places.
I use a magnet wand to remove and reinstall the toe clamp piece.

 

im using a gel cell its smaller battery i had to use strap