Is your 68 a WM-300 Flat Fender ? If Yes, a 4B series will fit no problem. Look at the link on my signiture. Look at the album 1943 Dodge Carryall. This is a WC-53 but for app practable purposes the engine compartment is exactly the same as a WDX or WM-300 PW. I am running a 4BTA CPL 857 driving a NV-4500 married to a NP-205. Its a great combo.
Paul