The thing I dislike most about a Nascar build is if the kit comes with a molded in exhaust system on the frame, it has to be removed. That is the way this old Johan kit came. Below are the steps I took to remove the exhaust. 
 
First I got out the sprue cutter. 
 
 
 
 
Then I got out the #11 exacto. 
 
 
 
Then I got out the bigger exacto (don't  know the number). 
 
 
 
The I got out the sanding stick with various sanding belts. 
 
 
 
Then I glued some styrene on the back of the holes caused by the removal of the mufflers. 
 
 
 
 
Then I puttied the voids where the mufflers were. 
 
 
 
 
I sanded the putty and primed the frame this afternoon. I will try to get pictures soon of the results.