Ok, for Body swapping. I rarely ever find the need to swap a body, unless I want one name on a differet car body...fo instance if I wanted a Black RX7 RZ, I would swap the body of the RS over the RZ.
But mostly, just start with the car that you want to use the body for and build from there.
Ok for Chassis swap, I will write up something here real quick.
Ok Obviously go to "Chassis" in MK's and click then right click, go to change parts(sorry I have to be specific just in case there are retards reading this....or someone who has never used the program)
Up will pop a new window listing every chassis in the game with the car owner name, weight and wheelbase for each. And on top of the screen(as with every part you change) will be the information for the car you are currently in.
Now you can scroll down and pick the chassis you want. Thats is the basic info...now to make your choice easier.
Click on the word "Wheelbase" above the list, and drag it to the left, so that it is in the first column. Then click the "Sort first column button at the bottom of the screen, now they are in order by length. Look at the car you are in, again right on top of the screen, and chek what it's wheelbase is. After finding that, scroll down to that length and find your car. Above and below your car will be the nearest length chassis to the one you are using. Try picking the lightest chassis within like15mm to the one you are using, and you shouldn't notice a visible difference on your vehicle. Unless you want to...look at your car beforehand and see of the wheelbase should be longer or shorter, like on the RGT....the wheelbase needs to be extended about 50mm or the (11 which needs to be shortend some.
Post, e-mail or message of you have further questions.