Sweetgrass MT to Anchorage travel at the end of March-how will roads be?

March is still very much part of the winter along most of the Alaska Highway. You can expect to encounter snow, ice, and temperatures well below freezing. According to Wikipedia climate data, the average daytime high in March in Whitehorse, Yukon is 31°F (-1°C) and the average overnight low is 11°F (-12°C), with temperatures below -40°F (-40°C) having been recorded.

The Milepost is an indispensable resource for driving to and in Alaska. Their advice on driving the Alaska Highway in winter (Question 10 in that FAQ) is worth heeding. The full text is extensive, but here are the important points (bolding mine):

Studded tires are legal in Alaska from Sept. 15–May 1 and from Oct. 1–April 30 in BC. Yukon’s rules on studded tires are in conjunction with Alaska and BC dates. BC also requires passenger vehicles to use winter tires on BC Highways from Oct. 1–March 31 and you may be delayed, have your car impounded or get fined if you are not in compliance. While studded tires aren’t essential, good winter tires are recommended (and required by BC) as snow and likely ice, is the norm. Road conditions should always be a concern and asking what’s up ahead as you drive along is prudent. Be willing to spend the night if the weather doesn’t cooperate with your travel plans. Crews in these northern areas are well equipped to handle winter weather and roads are quickly cleared and ready for traffic after storms.

Much of this route includes remote miles. ... Phone ahead for lodging to make sure you don’t arrive during an unexpected closure for the owners (even for year-round businesses). Have a block heater installed in your vehicle and keep your extension cord handy for plugging in if the weather is below zero. Keep all of your emergency road gear at the top of your load and within easy reach in an emergency. Be sure to have flares, jumper cables, a tow rope, all tire changing equipment and heavy outdoor gear for extreme temperatures, all handy just in case you need them. You don’t want to have to unpack your vehicle just to reach the car jack, if it is -20F and you get a flat. Above all, if you are stopped along the road and unable to fix your vehicle, set out emergency flares to alert traffic to your situation and then stay in your car till help comes. Keep sleeping bags, food and water handy to increase the likelihood of a comfortable wait.

(It may be difficult to find someone who knows how to install a block heater in Alabama. You might consider trying to find someone who can do it once you get further north, though this would require you to shorten or skip a day of driving. The same goes for winter tires or studded tires, come to think of it.)

The Milepost has another article on winter driving with additional tips. This list of tips for driving in the far north in winter is also worth considering. Finally, this travelogue from April 2017 might give you a picture of what conditions will be like along the road.

March is breakup season in Alaska and I assume you will encounter a good amount of slush/snow in Northern Canada as well. I would definitely pack chains and perhaps some sand.