Original English wheels (steel) 3 1/2" x 13 with no safety bead
Original Australian wheels (steel) 4" x 13 with single safety bead
Original English and Australian wire wheels 4 1/2" x 13 with no safety bead
Original tyres 620 x 13 cross plys
Optional radials — Dunlop SP41 or SP3’s in a size — 145 x 13. 145s are almost impossible to obtain now so the next size up, 155 x 13 is the better size and has the same radius as to 620 x 13 cross plys. 155s are best suited to 4 1/2" rims, there are other sizes with the same rolling radius — 165.75 x 13 which is a common size (Ford Laser etc.) and are suited to 4 1/2" and 5" rims. The other size is 175.70 x 13 and are suited to 5" and 5 1/2" rims. Any other size of tyre will be a problem.
5 1/2" rims and 175.70 x 13 tyres are at the limit for registration purposes as they almost protrude out too far at the front.
MKIV & 1500 Spittys are a different situation because the quarter panels are flared out much wider and have a different wheel tyre combination, starting with 4 1/2" rims and finishing up with 5" rims, but still no safety rims with 145 x 13 tyres on early MKIV’s finish up with 155 x 13 (usually Goodyear).
For me a 13" rim 5 1/2" wide with 175.70 x 13 tyres is the best all round combination with plenty of clearance. The best rim offset is 24mm positive.
It is possible to fit 14" rims 6" wide under late model Spittys, provided the wheels have a 101mm back space and tyre size being 175.65 x 14 or 185.60 x 14, but you are at the absolute limit and looks great.