The best way to build up a proper top line is good consistent riding, getting the horse to use its hind quarters properly by doing this the natural outline at the front end should come thus providing a better topline. Sorry if this sounds wrong but i'm not the best person at explaining what I mean, there is no quick fix to getting a proper top line, conditioning cubes just help with weight and basic condition.
you ant beat exercise for building topline...although their are products which are available such as blue chip and top spec...but make sure you do back it up with the appropriate exercise aswell....
Top spec feed balancer, working on the lunge in a pessoa and ridding regular doing plenty of hill work. Obviously the horse needs to build up gradually, good luck!!
The only way you can build topline is to exercise regularly and in a correct outline. Feeding correctly is important but the best way is exercise, you can't actually feed to get a true topline- this is a myth, another way feed companies con horse owners.