The absolute strongest assembly is having everything level in a line between the two mounting points, from the frame mount through to the tow bar mount. That way the towed vehicle trust is only in/out. IIRC standard common hitch height is 14 inches? If everythings set up to standard height, anything can tow anything.
In mounting a hitch to the frame of a vehicle, I always prefer bolts over welds, and grade 5 bolts over 8's. Grade 5's will bend and stretch and distort long before they fail, 8's will hold until they reach yield and then let go, snap. Just use oversize 5's to reach the same tensile strength your looking for. Rather than drill holes in the frame, often you can use clamps to clamp to the frame, not weakening it, but actually adding strength.