I have a '77 chevy van G10 with a 305, 3 speed transmission. I'm looking to get a new tranny for better highway gas mileage. If anyone knows what I need I'd love to know. I'm looking for a 4 speed overdrive tranny but not sure wich models are going to fit. I'm reading on other website that maybe a TH350 or TH700 R4

thanks all