Converting 68 Valiant Rear to use 15" wheels
What is your eventual intent for the car? Do you plan on keeping it stock 225 slanty, or do you have plans for a V8. If you intend to keep it a slanty but want the option of disc brakes, take a look at the "scarebird" front disc brake conversion kit. It uses the stock existing drum brake spindles and hubs on the valiant, and retains the small 5 bolt 4" bolt circle. It uses toyota previa rotors that are re drilled to the factory small bolt pattern, and uses chevy celebrity rotors. It will require an upsize to at minimum 14" rim, but will work with your existing bolt pattern. More modern stuff for the small bolt pattern, makes spare parts a little easier to find than the old Kelsey hayes small bolt pattern discs that these cars had from the factory from I think 1965-1972. Go this route, and ya sure you will have to upsize the rims to clear in the front, but you may want to upsize all 4 and the spare. This way when you inevitably get a flat tire, your spare will fit the front or the back.
The big bolt pattern kit, your mentioning now sets you up with a larger 5 bolt 4&1/2" bolt circle. Requiring at minimum a 15" wheel. If you do not do the same thing with the rear end, your looking at carrying 2 different spare tires just in case, or just take your chances your not gonna get a flat.
Honestly if your gonna leave it stock, I'd hunt for five 14" steel rims that will fit your 5 bolt four inch bolt pattern, get the scarebird kit, and just enjoy the car. You can always collect up what it needs for a V8 conversion while enjoying it, including storing away the big bolt pattern kit the seller wants to let go with it as the first piece to the puzzle. Theres always buyers out there who would want the scarebird kit and 14s when you dont need em anymore.