First, the new transmission must be compatible with the engine and drivetrain of the older car. This includes the type of transmission (manual or automatic), the number of gears, and the gear ratios. If the new transmission is not compatible, it may not be able to function properly or at all.
Second, the installation of a new transmission may require modifications to the car's chassis and driveline. These modifications may include changing the transmission mount, driveshaft, and other components. In some cases, it may also be necessary to modify the car's computer or electrical system to accommodate the new transmission.
Third, the cost of installing a new transmission can be high, depending on the type of transmission and the extent of the modifications required. It is important to factor in the cost of labor, parts, and fluids when considering whether or not to install a new transmission in an older car.
Overall, it is possible to put a new transmission into an older car, but it is important to carefully consider the compatibility, modifications required, and cost involved before making a decision.