Early 64/65 A833 to later gear conversion?

I have a 1965 A833 with the 3.09 first gear, I want to convert it to the lower numerically 1st gear of the 66+ cars. Is this possible? Will I have to gut the entire case and start over with every gear set and input shaft from a donor trans? Oh and I want to convert it to non-ball/trunnions as well. Thanks in advance I'm new to these fancy 4 spd thingies...