I have run the Michelin Taxi tyres for a number on years on my 400. I keep as the spare the 3.5" x16 dunlop rim as it fits inside the rear cover with an old Michelin x on it.
If I am honest the Taxi style tyres for me take away part of the ride smoothness & flexibility that the old 5.5"(on 3.5" rims) Michelin x had, but were a lot cheaper but very "stiff" and noisy.
I had to buy a set of 4.5" rims from Brian May(or was it 4"?-I cant recall but they are 401 rims) to accommodate the Michelin taxi tyres.
I still have the 4 remaining 3.5" rims and have contemplated switching back to the Michelin X but cost has prevented me.
The other day I saw this 400 on car and classic sporting 5.5" Blockeys not cheap but good tyres
https://www.carandclassic.com/car/C1666765