It's certainly a good idea to take the measurements that Dennis described. One especially critical one is the location of the bolt that ties the subframe to the frame rail, especially if the inner frame rail reinforcement is being replaced also (in which case you lose your bolt hole location).
Having done this repair several times, I would recommend supporting the front of the car on the frame rails themselves. Assuming you're working on the driver's side, a jack stand can be positioned to support the passenger side where the front jack point is, and the other one can be further back, beyond the damaged area, preferably at the point under the front seatback where it joins another transverse rail. Use a piece of wood to distribute the load. When the repair piece is fitted satisfactorily and clamped in position, jack the car and move the jack stand to the same position as the passenger side so that the repair piece is actually supporting that side of the car. Then make the necessary welds. This will tend to give the frame a slightly positive arch, which evens out with the wheels on the ground.
Another tip: That bolt which ties the rear arm of the subframe to the frame rails is usually quite stuck. If it doesn't want to come out it's better to disconnect the arm from the subframe. Remove the nut from the lower control arm bolt, the bolt that attaches the steering rack, and disconnect the stabilizer bar's rear mounts. By prying a little between the arm and the subframe you can clear the LCA's bolt and swing it free. Then cut off the head of the stuck bolt and drive it through, along with the arm. Once off of the car, you can remove the bolt from the rubber bushing's sleeve much easier on a work bench. The down side is that you have to buy a new bolt, but you would probably destroy more than that fighting with it on the car.
Good luck.
