Heavier Springs will not cause any issues as long as you re-align the front end again after they are installed. Now this is if they are original or replacement springs that are recommended for your car. I put 396 Springs in my '67 283 car years ago and it went from a sagging 1 or 2 Inch under to a nice 1 or 2 Inch over. I also ran 14 X 6's in the front of that car and it all worked out great.
In my opinion, the 15 X 8's in the Fronts are too much. I ran 14 X 7's in the front of my '56 and did not like them. Went back to 6's and liked it a whole lot better.
If you still have issues, bring it to a good, well-known Front End Shop. Have them check the Ball-Joints, Idler Arm, Pitman Arm, Tie Rod Ends, etc as well as the Alignment.
Wheel Bearings too.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America