If you slip or the spring breaks are your face and knuckles and time off work injured worth saving $100? If you don't measure right and install the wrong size spring the door will not function properly and it will damage your garage door opener.
Your garage door needs a reinforcement strut across the top section and the opener is mounted to high above the door. Lower the opener and add a strut to prevent the opener from bending the top panel of the door. Other than that good job.
its easy ive done it many times for customers that cant get into their garage.