I'm part way through building a full chassis semi dump trailer with full wireless control to match my other trailers. I'm using a screw drive with a 540 motor and 100:1 gear reduction, powered by conventional 7.2V pack. I'm thinking this will tip pretty much any load.....but until I finish the build no way to really tell without some hard-*** math!
if you are struggling for a gear motor, try
www.technobotsonline.com. They have the biggest range of gearmotors I can find - everything from micro 3v motors up to 45mm diameter 15V drives. Every available motor can be purchased with your choice of reduction gearbox, from their list of options. Good value too.
tip angle in the range 48 - 53 degrees should do the trick....