Here’s the Facts:
Both the M105 trailer springs and the M35A2 fronts have 12 leaves and are 2.5” wide but the M35A2 spring pack is 4.5” thick where the M105 trailer pack (without helper pack) is 4.0”. The exact rate of each spring pack is unknown, but it is safe to assume the M35A2 springs can carry more load. If you also use the 1.5” thick helper pack from the M105 trailer (you’ll have to mount the stops on the frame too) the M105 trailer setup should have more load capacity than the M35A2 fronts.
The M35A2 hangers mount under the frame, the M105 trailer hangers are on the side. You’ll need about a 4” lift block to use the M105 trailer springs.
Bottom line is; if you have a M105 trailer, so you can use the bed, you might as well use the springs too.