Very few alternative seat bases will fit because the gap between the transmission tunnel and inner sill area is so narrow. It's quite deceptive because the seat backs look a standard width. If you adapt runners to tilt the whole seat forward, they will need a locking mechanism, which became the standard years ago. Also bear in mind that your chassis crossmembers ahead of the seat and behind it have been drilled for the current runners, and so you may need to drill extra holes with replacement runners, which is not ideal. So sourcing the correct tilting backrest seats may be expensive, but is the simplest route.
