Wallmart sells a kit like that, yet, I paid $800 to an a/c shop in Chico to convert an 87 Chevy Duellay I had as a ranch truck. I was told that "you get what you pay for". If you want it done right, it will cost. If you want a quick job that will cost you in 6 months or so, buy the $39 kit and do it yourself. Then later, pay for a complete new system. I might add, the 87 is still blowing cold yet, the guy across the road, went the other route and after 8 months, had it done right and admitted that he should have spent the money up front as now he had to replace more parts.
Just my 4 1/2 cents worth

For the record, I just was quoted $950 to convert my Corvette. No more $75 a pound R-12 every year. The vette leaked about 1.5 pounds a year. It now needs a recharge and I am just gonna have it done right as I will NEVER sell this car. I just hit 70k miles and its almost 24 years old.
It just gets better with age! Just like our trucks