I recently did this on my 92 cab - I actually bought the rear cushions from John - thanks pal!
The only thing I would add is that the lap belts dont fit super tightly on my 6 year olds and it doesnt seem safe to let them ride back there yet - I sourced a place in England that makes custom 3 point belts and they have photos of a completed project for a 944 cabriolet.
Im still waiting on the belts but because the photos dont show where the mounting points were created for the shoulder belt Im not sure how its going to turn out. In the coupes the shoulder belt attached on the pillar, we might need to construct something on our convertibles.