I've had 205 60 15s under a 144, and these tyres usually go onto a 7 inch wide rim.
Standard 140 offset is 40 with the 5.5 inch standard wheels (although a '68 may have had 5 inch rims).
You will likely - at minimum - need to roll the rear wheel arch inner guard lips to avoid chewing the outsides. Turbobricks has how-tos for more advanced chopping of the rear wheel arch lips.
Nug's link is worth checking out to work out your space.