I use the thinnest and longest to put engines in the fusilage. Just attach a 1 piece lenght 5 width and hieght .5 and then put the engines on that and take the length down. The engines move with the middle join on the fusilage and end up almost 2/3 the way in. This is my technique for engines and such. :)

You put a part on the block at it's smallest state then increase the fueselodge part size