Consider room for shoulder width when fitting buckets. I suppose if there is no top and you sit high enough, the top of the body can be an armrest but they can't be too close together. Not as pretty but it is hard to beat a flat bench. You are where you are with buckets.
You could have essentially flat bench for touring and a drop in center divider (like rigid curtains but an upholstered panel) for autocross.
https://www.kartpartsdepot.com/Go_Kart_Seats_s/1887.htm
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
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360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12