So what is the trick to get the front fenders off. I am used to seeing the bolt on type. I am looking at a slant nose kit and was checking how easy/hard to swap. are they spot welded at the seams or what?

These fenders must be mostly cut off for the fiberglass slant nose jobs (Chalon or Sheridian?)
There are some who can cut the signal light pontoons and roll or add metal to keep the fender all metal. You can search and ask ghiaspecialties.com.

Will cutting off fenders make the body structure weaker or does they matter?

Might make it weeker, but Im guessing you would be putting some fenders back on? If you do weld some fenders back on then I dont think the strength of the car would change at all.