May I ask why the molasses? It is just adding sugar to the feed, which may help gain weight, but it is going to be quite unhealthy, and it would take a lot to help gain weight. Now I use it as a occasional treat, but it is just full of sugar, and it doesn't do much, you could add some water instead to make it a little mushier. Also, I recommend feeding rice bran, slowly introducing that into the diet, as it is high in fat. Fat is very efficient in gaining weight in horses - and people!- and usually is a bit cheaper than feeds such as Sweet Feed, that have lots of sugar (molasses).
And don't cut back on protein, malnourished horses need more than normal!
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown