Driving conditions in April can be dicey - black ice at night/mornings, frost heaves, other breakup woes - but if it was me I'd drive to Glennallen and turn right. The Richardson into Valdez is arguably one of the most beautiful drives in Alaska and (IMO of course) a much better choice than the Glenn - Richardson - Parks loop or some other alternative. Yes, you'd have to retrace your steps coming back, but that's not much of a hardship; pretty much nonstop scenery. (Or you could, of course, put the car on the ferry in Valdez and cross PWS to Whittier - a superb but more expensive alternative.)