http://ssdi.genealogy.rootsweb.com/cgi-bin/ssdi.cgihttp://ssdi.genealogy.rootsweb.com/cgi-bin/ssdi.cgi has 12 William Clellands who have died. None of them look like your grandfather. There are 359 people named Clelland on the data base, to give you and idea of how popular the name is.
If you know when and where your grandfather died, you can try for his obituary, either by a post on a county forum or by writing to the library. Either way someone will be looking at microfilm, so you need to tell them exactly which day he died. I look up obits in my county; I can scan about a week before I get seasick and have to stop.
The same for your grandmother Helen.
Obituaries usually list surviviors, which will give you the first names of your aunts and uncles.
If your father is still alive, coherent and speaking to you, he would be a gold mine of information. Ask him about his parents. If he is dead, senile or estranged I apologize for mentioning the subject.
You usually have to get back to the 1800's before you find a lot of data on the Internet.