Just keep in mind, until you go to an overdrive trans, the powerglide and th350 are both 1 to 1 in drive. What that means not much changes except another gear, or shift point on the 350. As mentioned the 350 will bolt right in, however it will not change much. There are some slight gearing differences, but not like if you changed over to a 700. And of course cruising down the highway is where you will see another change. If you are happy overall with the powerglide and just have an opportunity to get a trans in better condition, then go for it. If you want a real change, move to a 700r4.