My advice is to build it on the mild side. Keep the compression at 10:1 or a bit less and use a standard port head such as the TF 240. I'd keep the cam on the small side too, maybe something in the 250 range. Your chassis isn't going to handle a lot of power so no reason to build something that isn't any fun to drive. Just build a big smooth torque monster and enjoy it. I'm not even sure I'd use a solid flat tappet, why not a hyd flat tappet? You aren't going to spin that thing past 6000 rpm without killing yourself so a hyd flat tappet will work just fine.