Allison and manual usually don't go together. Tulsa Truck is one place, they do alot of 4x4 conversions for the OE big name truck manufacturers. Be sitting down when they give you a price, they might not do your truck too since it is old and they might not have the engineering done for a conversion on it. You can go the heavy duty junkyard route and source the front axle and transfercase on your own, be prepared for fabricating or paying a good shop to do it for you. Find out what your rear axle ratio is and if you will need to change it if you're going bigger tires. Measure the front spring width and the distance between the front spring centers before you start shopping. Also most medium and heavy duty trucks have the same width so get the transfercase mounts and crossmembers with the transfercase as it will save you time and money over fabricating your own.