The other option is the one used by Barney & Smith when shipping passenger cars to the Carson & Colorado in 1881. The cars were shipped on standard gauge trucks (which B&S said the Southern Pacific would purchase) and the narrow gauge trucks were on standard gauge flat cars.
I have seen builder's photos of ACF cars built for Ohio River & Western being shipped on standard gauge arch-bar trucks with the narrow gauge passenger trucks on an adjacent flat car.
There are also photos of F&CC and N-C-O car bodies (without their trucks) blocked up on flat cars for shipment.
Brian Norden