The reason your seats lean towards each other is because you're probably missing the reinforcement plates for the inner bolts OR the large spaces haven't been installed. These would raise that side of these seats by about 1/2".

Also, the outer holes were different for bench and bucket cars. So if one of the owners had converted to buckets like I did, he would have had to drill the extra two holes. And I never realized those inner brackets existed for years so I drove around with my seats slightly tilted also !!