brassicas prefer heavy topsoil, break the soil to a reasonable tilth and plant the plants up to the growing tip, im assuming your planting plugs or seedlings? then once the plants are growing, topdress with the manure.
brassicas have such a small root system for the size of the plant that you try to make the plant feel stable by planting deep, then encourage the rootball by top dressing. manure rich soil will make them grow too lushly to be strong for the winter.
another tip is to soak them in compost tea as you plant them.
and i agree brassicas are the best,they provide the most lb / ft2