Wrecking yard. Find a bad frame on a half ton and torch the crossmember out with the frame rails and rivets in tact after you measure the mounting location. Take it home and get the rivets out. You'll likely find that the rivets are a bit larger than 3/8. Try a 10mm or 7/16 in the hole. Mark your hole location and drill! Install with grade 8 bolts. Some will just measure an existing truck and weld some channel or angle iron in its place and attach some shock mounts. Not recommended, though. Always bolt instead of weld. Welds create stress and crack.
