Cutting the frame and welding back together is a piece of cake, if you do it right.
The way I do it is where it needs to be lengthened at, mark your frame, both sides, with a tape. Get the frame as level as you can. If your frame is level with the slab, use a large carpenters square, make your marks inside and outside the frame. (I use my chassis jig for this)
From this line inside the frame, I measure 2" forward I make a line, then 2" back, I make another line. I use silverstreak so the lines won't come off easily, until I want them to.
Get a couple of pieces of 2x3 tubing, about 4' long, but they must be the same length. The marks 4" apart inside the frame, weld in a piece of 2 x 2 x 3/16's tubing in at both places. This will keep things from spreading and all, when things are cut. Now, you have 2 pieces of tubing welded 4" apart, with a mark in between them. Get your 4 1/2" grinder with a slicer wheel on it, cut across the bottom of your frame on both sides. Use either a file or a metal conditioning pad on your air tool to deburr your cut.
Then, with 2 big C-Clamps, put your 4' long pieces under your frame, centered with the cut line in the middle. Put the clamps toward the firewall side, 1 right above to cross bar you welded in, and the other about 6 inches in from the end of the bottom tube. Do this on both sides, and exactly the same way on the front section of the frame. Make sure that the rear clamps you did first are really tight, and that the lower tubing is straight with the frame.
Now, with your slicer wheel, cut all around your frame tubes on each side. Now, you can loosen the front frame c-clamps, slide it forward carefully on top of that lower tube, add your piece in, clamp the front end down again good and tight and straight then tack things back really well. Doublecheck for squareness and being straightness....thats the reason the the lower tubes being square and straight, for a good visual reference.
Put a good, strong weld on top, across both inside and outside, but don't weld up completely yet. Now, drop your lower pieces, run a weld across both bottom frame rails....on both sides. Gotta jump around alittle to keep uniform heat in the joint. If you welded it correctly, very little will be sticking up above the tubing, get a flapwheel on your 4 1/2" grinder and smooth the weld a little, for looks.
Do all your other stuff, touch up the paint, put her back together and get ready to go cruisin'