You can try this:

.blog #secondary,
.single-post #secondary {
width: 32.9149%;
}
.blog #primary,
.single-post #primary {
width: 65.9575%;
}
.blog .entry-content,
.single-post .entry-content {
float:left;
width:605px;
overflow: hidden;
}
.blog h1,
.single-post h1 {
font-size: 20px;
font-size: 2.0rem;
}

@media only screen and (max-width: 800px){
.blog #primary,
.single-post #primary {
width: 100%
}
}