Moving rear springs.....

Nathan here, got a 69 valiant in need of spring reloc. kit, Im 20 yrs old with a bit of fab. experience.. floor pans frame connectors .. and such things. and to honestly tell yall, im kinda freakin out b/c if when i cut and well the boxes in, if its not precise, the damn car will dogtrach down the road and look/handle like hammered ****. need some quick advice or the most accurate way to do this. read a post but the guy drew lines on the concrete with chalk.. thats to backwoods for me , plus ive got seamsealer and undercoating all over the floor from the floor plans.. need some help my fellow mopar brothers.. :cussing: