Since your question pertains specifically to how to stretch a motorcycle frame, I'll focus on that only:
The frame has several tubes that connect the neck to the rear axle plate. If you are lengthening the frame then all of these tubes will have to be adjusted in order to make it all work correctly. Keep in mind that the backbone is at an angle that connects the neck to the rear axle plate. Lengthening this distance is going to force the angle to change. Be careful when making this change because it will affect the engine mount at the top of your engine to the backbone.
The vertical portion of the downrails should be the same. The horizontal part of the down rails can be extended to accommodate the length you want to stretch. The stretch described will open up the center cavity for your engine and transmission.
If you are interested in keeping the engine and transmission cavity the same and still want to stretch the frame, you can do this by stretching only the rear part of the frame. The tubes that come up from the down rails and connect to the rear axle plate can be stretched. Again the backbone will need to be adjusted to make sure all will fit up correctly.
The rear fork should not be too complicated to lengthen to match what was extended on the bottom side.
Frame Stretching Basics:
Most street touring bikes and cruisers have the foot pegs and the gear shifter positioned in such a way so that you have to stretch your legs to reach them. They designed with comfort in mind. But they are also stable at higher speeds.
Conversely, crotch rockets are all cramped up for arrow dynamics.
Some bikes have poor design where you find that your tail bone is getting a rough ride (no jokes!)
Also, you'll find that cruisers have the handlebars forward so that you have to reach for them a bit, but in a comfortable position.
A chopper is basically a cruiser that has been raked and given a more bad-ass look to it. There are many variations that come to mind but you've seen choppers with big v-twin engines, ape hangers, long forks, and stretched frames.
The main reason a chopper looks like a chopper is usually because the frame has been stretched. This is accomplished in several ways but one of the technique to get a chopper look to your cruiser is to take the downtube part of the frame and extend it. You can also longer forks.
This is sometimes done for comfort, but often it's done for looks alone.
The better alternative to stretching a frame of a cruiser is to build a chopper frame that has all the geometry worked out ahead of time.
Stretching a frame is accomplished by lengthening the bottom rails and the wishbones, and extending the top tube and downtubes in order to change the neck angle.
This is an example a stretch made in the rear portion of a frame. As you can see the axle has shifted upward resulting in a lower chopper.
You can achieve a chopper look by stretching the front end of your frame. You accomplish this by using two the techniques I just mentioned. You will be extending the frame without but not changing the rake of your chopper.