Ive always had good luck by just lining up the trunion on the outside of the receiver then marking the holes with a bit or punch.
Just make sure that when u drill its actually were the mark is, not were u think u see a mark, i now have an extra hole in the flat i bent last night. I'll fill it when i plug weld the rails in
Make a jig that holds the trunion at the tail end and won't interfere with slipping on the receiver. Then clamp the jig in a vise and put it on the drill press. With the vise on the drill press align a trunion hole so the drill will drop straght through the hole then clamp the vise down. Slip on the receiver to where you want it to sit on the trunion, clamp it down, and drill through it. Repeat the drill bit alignment for the other trunion hole then slip on the receiver and drop a rivet through the previous hole you just drilled to keep it aligned. Clamp the receiver and drill the other hole.