GM's factory roller setup uses a sheetmetal 'spider' plate that helps secure the roller lifters in their bores.
Retro-Fit kits use a "tie bar" at the top of the lifter
GM Factory cams have a 'stepped nose' design which uses a thrust retainer plate
Retro Fit kits normally use a thrust button that is trapped against the face of the cam by the timing cover (you shoould use a sturdier timing cover or braze a piece of flat stock in the cover as a reinforcement.
Make sure you measure your pushrod length as it will change.