Either you can go for B.tech in mechanical engineering or you can apply for government jobs based on your polytechnic degree a lot of vacancy exists in this.

You can also consider taking up B.tech while you look for government jobs, once you get a job put B.tech on hold for a year. Once you complete a year or two in the job take a long leave and finish your B.tech. Post you complete your B.tech you will be promoted to assistant engineer from junior engineer. This is another career path to ensure speedy growth.