I'll throw my $0.02 in : A12 with some sort of Twin Cam head and Fuel injection - Gearbox diffinetly sequential. My sports racer was motorcycle engine powered and sequentila box is much nicer , left foot brake and go up and down through the box without having to use clutch. Maybe adapt Honda Goldwing gear box to A12. Beam axle is fine but get rid of leaf spring rear end and go to 4 link.