I've been down a similar road and used the sport seats from a late E24 in my own coupe - I think the seats are largely the same a the E30 ones (share a lot of part numbers etc.) and yes, the bases are integrated into the seats, so not an easy job to get into a coupe. Some cutting and welding was needed, I decided to hack the seat bases rather than the structure of the coupe - so the original seats could go back in without any issue. Much more comfortable, they don't look out of place and well worth doing.