They attatch to the holes for the original lap belts, using the bolts you take out of the stock belts.

then where do the lap belts attach to? the same place?

factory seats could be a problem (as shown above) only because they will not keep the shoulder harneses in postion in a hard crash.

They would tend to roll off your shoulder and you would have no shoulder harness = BAD

A seat with separate headrests (like the Design 2000 seats I used to have) or another seat with holes will work fine. you need to find a balance between what you want and what your dad wants....

Here is a photo of what the design 2000 seats kinda looked like.

http://i226.photobucket.com/albums/dd320/markgoze/seats/recaro%20njoy/DSC_5611.jpg

The lap belts attach to the factory locations the shoulder belts attach to the bolts for the rear struts. The shoulder belts go under the headrests between the two posts that hold the headrest up.

http://cgi.ebay.com/ebaymotors/Recaro-Seats-Blue-VW-Helios-Retro-RARE_W0QQcmdZViewItemQQ_trkparmsZ72Q3a543Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318Q7c301Q3a0Q7c293Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem270324529849QQitemZ270324529849QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

A seat that is too wide with shoulder bolsters or a seat that is thick on the bottom will not fit well in your Z.

For minimum cost and intrusion, check out the G-Loc. I've used them while driving student's cars at track events and have found them quite effective.