I have a 64 GP with a 389 it has the origial trans. The transmission needs to be rebuilded. Before I have it rebuild, is there a better trans that i can install. If so How much work is it.  

What trans do you have now? Auto, 3-spd manual, 4spd manual? Let me know and we'll go from there.  

If you have an auto, it's the 4-speed RotoHydramatic. They were on fullsize Pontiacs prior to '65. If "R" is at the bottom of the shift pattern, then this is the tranny you have.

You have a few options: 1)Rebuild the original. You might have problems getting parts, and some tranny shops won't touch it. 2) Replace it with a TH350. This tranny will bolt right in, but you'll need to move the starter from the bellhousing to the block, add a cable kickdown, and you might have to alter the driveshaft length. And of course, the shift pattern will be different. 3) Replace it with a TH400. I think it'll bolt in, but I believe you have to swap the crossmember. You might also have to shorten the driveshaft.

I had the same problem with my '64 Bonneville. I replaced the old tranny with a TH350 from TCI. I believe all full-size Pontiac engine blocks in '64 have a boss for mounting the starter, so you just have to drill two holes.  

The transmission is a 3 speed automatic.How hard is it to install a th350 or th400.?  

I'll pass on this one as Deadweasel101 seems to have a handle on it, having done the swap before. It's a bit out of my area of comfort as I don't have much experience with Pontiacs prior tp the late sixties. Best to ask the guy who has done it.  

To do the swap, you will have to drop the driveshaft and remove the old transmission, torque converter, and flexplate. Check to see if your 389 block has the boss for a block-mounted starter; if it does, drill two holes for the starter. You will need a new flexplate to be compatible with the block-mounted starter; they are available from TCI, which also makes a mini-starter that will fit.

If you install the TH-350 as I did, it should just bolt right in. The linkage will hook up fine, install a universal transmission mount to support the transmission, and alter the driveshaft to fit. I didn't have good luck with the TCI StreetMaster; it leaked fluid from the pan and speedometer housing, and the planetaries failed after 6 months. Get a good BOP T-350 from a good tranny shop. Install the proper torque converter, bolt the transmission to the engine block, connect the linkage, and install the starter. You will need to install a cable kickdown from the transmission to the carb linkage; Lokar should make them. It's a bit of work, but it can be done by an amateur.

The T-400 swap might be different; I haven't tried it, but I understand you have to change the crossmember. That's probably a bit more work.