Yep, I'm here with my '74.
I'd go with the 400 unless you wanna stick small block really badly. My main question would be is are you gonna go with a bed on it? My plan so far has been to cut my bed floor out and replace it with a sheet of aluminum to lose weigh. you can also wrangle you up some 'glass fenders and a hood like I wanna do. If you have any questions about your truck, please feel free to ask. alot of us on here own one surprisingly