If you are having problems with your freehand cutting get one of these gadgets
http://www.ebay.co.uk/itm/4-1-2-ANGL...8333%26ps%3D54 The do them for 9" grinders as well. Used with a thin disc it would do all you need. Its not a good idea to gap craft the joints because the weld will pull and cause distortion making it harder to keep everything in check. Practice weld on all the off cuts of material before you start on the chassis, good luck.
Bon