Chevy BlazerK5 truck tax despite backbench???

Hello! Now wants to buy me a Chevy Blazer K5 from 1983. The car is registered as a truck, but without a back seat. I want to drive it with truck approval and back seat. I heard that this works. Can someone tell me how to replace it and what else to consider when buying a K5? Can you recommend a K5 from the repair effort and reliability??? Thank you for answers. Your Max