This depends on what era car you wish to build, if its a 50's or later car the easy way is to buy a second kit that has a pro street chassies and modify that to fit.
The other way is to cut out the original kit chassies wheel hump so that the tyre you wish to use fits well, then build the 'tub' back up using plastic card, you will have to then make a new rear chassies to hold the rear end in, you can use your imagination there are so many way a rear end is set up!.
For a Hotrod, (I build these in all scales including 1:1) This depends if it has the floor and chassies molded in one or separate.
With a separate one cut the chassies in two forward of the wheel openings, build a cross brace so that the chassies is now shortened and enclosed. Now calculate how narrow the chassies needs to be to accommodate the rear tyres, narrow the remanning chassies section to that and glue to the forward section, vola narrowed chassies, build the tub's from sheet styrene as before.
If it has the floor molded in this is harder, I usually cut the whole floor from the chassies and remake the floor from sheet styrene, then narrow as per the last way!
If I knew how to add pics I would add some accompanying illustrations for you!
Hope this helped!
the Doc.