Work out a cost of ownership comparison and show it to him. figure fuel costs, oil changes etc. Then calculate the average resale estimate after you have owned the vehicle for about 5 years and subtract it from the initial purchase price, divide it by five and add it to the cost for each vehicle (deprecation).
Call your insurance company and get insurance quotes for both vehicles.
Call a tag agency and get the cost of the tag for each.
If you present a well thought out argument showing the pros and cons for each vehicle, I mean have charts and spreadsheets to show him... it will be very hard for him to say no. Dad's love that kind of shit.
And as for the injector argument...I just looked it up and new stock injectors are a whopping $60 bucks each so to replace all six would be about the same as buying a new Holley 600 cfm carb.
http://www.cumminspartsdirect.com/