Now that you’ve got your tubing cut, we can start on the main tapers. The drawing below has the dimensions. I would recommend using a cut off wheel in a die grinder, it seems to give you the control you need to make straight cuts. If you don’t have an air compressor, a sabre saw with a 24 tooth blade will work. Using a plasma cutter seems to put a lot of heat in the metal causing it to warp. The relief cut to the outside of the rail is ¼ inch at the end of the long cut.
After all the cuts are made, pull the bottom section up to meet the top – be sure to keep a 2 inch opening for the front crossmember. You might have to adjust the relief cut to allow the two sections to come together. When everything lines up, put a tack at the front of the long cut. By stacking the two front rails on top of each other, you can check to be sure they match. This is very important. You want one side to be an exact copy of the other.
Before you begin welding, take one of the rails and flip it end for end. Now clamp the two together as shown. This will keep the distortion to a minimum. I prefer to mig weld the tapers, so I leave a 1/16” gap to ensure penetration. It is also a very good idea to skip weld these seams. Start on one side and run a 1 inch weld every 3 inches. Now do the second side of the same tube. Now do the same with the second tube. Then back to the first tube. You want the tube to be cool enough that you can lay your hand on the tube before you start the second set of welds. Run your welds up to the previous welds from 1 inch behind them. In other words, you want the second welds to end where you started the first welds. Now keep switching tubes until all 4 seams are welded. Let the tube get completely cool before taking them apart.