I'd worry more about finding an accurate bullet than if it's 100,120,130, or 140 grains and actually try them at longer ranges to see which ones are best.
Your concern about initial velocity isn't going to change much for you at longer ranges. Some heavier high ballistic bullets suggested by Northman999 may prove to be your best bet. They may start off slower, but they will keep their velocity better due to their higher ballistic coefficient and have less wind drift.
Here is a ballistic table on the 6.5x55. You'll notice that at 400 yards there can be very little difference in bullet drop between the lighter and heavier bullets providing you pick a bullet designed for longer ranges.
h ttp://www.rifleshootermag.com/ballistics/65_55_swedish.html