would puting in heavy duty rear coilsprings raise the vehicle height or just make it stiffer?

That depends on where you plan on getting these springs. Have you found a supplier yet? They are the ones to ask.

i have found a supplier but they are not sure in genereal would heavy load springs raise the car height
?

That's determined by the individual design of the springs.

you dont give much information do you?

I don't know what you're using for springs so I certainly don't know what the result will be. I advised you where to get that info.