I wouldn't fill the old ones. Get a new replacement, or use the puck stack for the front/side mount.
For the rear, pucks are not really an easy option. I bought a new replacement and filled with 08609 window weld. Be aware that replacing the front and rear mounts with solid versions will increase vibration into the cabin, especially through the steering wheel. Filling the rear produces the most vibration; pucks up front a little less.
Links to front and rear mount write-ups are on page 1 of this thread.
'98 SC Riviera 268k miles 298 HP/370 TQ 0-60: 5.79s ET: 13.97 @ 99.28 4087 lb 20.1 avg MPG Nelson Ledges Lap: 1:30
3.4" pulley AL104 plugs 180Ί t-stat FWI w/K&N 1.9:1 rockers OR pushrods LS6 valve springs SLP headers ZZP fuel rails
KYB GR2 struts MaxAir shocks Addco sway bars UMI bushings GM STB Enkei 18" EV5s w/ Dunlop DZ101s F-body calipers
EBC bluestuff/Hawk HP plus SS lines Brembo slotted discs DHP tuned Aeroforce Hidden Hitch
'05 GTO 49k miles 0-60: 4.8s 16.9 avg MPG Nelson Ledges Lap: 1:26