The twin nostrils were the 1964/5 first design P1006 design. I am certain that I heared in the past that the MKIII's nostril spacing or depth was different to a MKI. So those with KVA MK1 bodywork beware. Because all Ken Attwells moulds were taken from a MKIII.
In fact the 1966 MKIIa's had a single vent divided by a thin metal plate on the Shelby and Alan Mann's cars. Holman and Moody MKIIa cars not having this dividing plate.
As I understood it the single vent was tried on the M1 Mirage in 1967 prior to Le Mans. The Mirage used the single vent in its Spa victory and subsequently including Le Mans. The single vent was also used on the MKIIb's. The spare wheel was at the back seating upright on the MKIIb's so the wheel change in Ian's comments would not be necessary in the case of the MKIIb's.
Which brings us back to the MK1 the 1966 Le Mans there were 5 MK1 GT40's entered all having twin nostrils P1017 Comstock, P1040 Filipinetti, P1007 Ford France, P1038 Essex Wire and P1001 Essex Wire.
For the 1967 Le Mans race 3 MK1 GT40's took part twin nostrils P1020 Ford France,
Single vent P1042 Filipinetti, P1026 JWAE.
By the 1968 Le Mans all MK1 GT40 P1074, P1075, P1076 all JWAE cars P1078 Strathaven Ltd, P1079 C Dubois all had single vents.
In the 1969 Le Mans 7 GT40 MK1's P1075, P1076 both JWA, P1081 D.A.Z, P1009 M Guthrie/Alan Mann, P1033 E.S.C.A., P1082 Ford France all had single vents and the only GT40 Mk1 with twin nostrils was P1010 E.P.Sadler.
Since Ian comments apply to Le Mans I cannot ascertain to why single or twin vents would affect the extraction of the spare wheel especially as the GT40 production chassis took this into account. It had a half circular wheel well already incorporated in the production chassis run. Indeed the only car that had a bulge made to accommodate the spare wheel in a different position in it's front hatch that comes to mind was AMR/2 Paul Hawkins famous lightweight and that was a modified single vent.
Regards Allan