1988 G20 van conversion
from LANDMARK needs running board/ fender flares
cracked-body shop says theyre plastic not fiberglass.
what can i do to repair?

 

How about jb weld

 

What about putting up a picture for us to see what kind of damage you have....
I am sure all the ones I have seen at the junk yard are fiberglass...
I may be wrong I have a 2003 GMC Sierra and the Sportside bed was told
to me that it was Plastic.. not Fiberglass... So maybe it is plastic...
but I think a picture would be great to see your damage

 

You may be able to reinforce from behind with fiberglass.....as long as the resin will bond. There are diff types.