You don’t need to relocate the springs the full 3” to put them in the frame rails and do the mini tub for 18x10’s.
But you should get a 1/2” spring hanger/shackle offset and move the perches to 42” c-c.
I’ve got 18x9’s and 18x10’s on my Duster with a 68-70 b body rear with 275/35/18’s up front and 295/35/18’s our back. But I also have the 1/2” spring offset and I trimmed about a 1/2” off of the quarter lip to do it.
