3 pt belts don't fit well in these big back window trucks with a 2'' wide b pillar
i looked at all sorts when i chose my seats, even to adding structure in the b pillar
it wouldn't have looked right, so i just use lap belts.
try looking into convertible seats, most have the shoulder belt in the seat back
but also most are buckets with a console.
some gm truck seats have the intergrated shoulder belt in the seat, but probably too wide for your use.
even then they would not have a shoulder belt in the middle seat.