How difficult to replace the Powerglide with a TH350 in a 65 Chevelle column shift. Any mods req'd to the rear mount, shift linkage etc?. I'm sure the driveshaft will need to be re-done. If there are dummy-proof conversion kits available, where can they be purchased?

the trans bolts right in- even the driveshaft will work perfect.
might need to do a little bit of figuring on the shiftl inkage, but it isn't too hard.

I used a 350 out of a 72. For that I needed the lower shift linkage from the 72 but the upper shift rod from my 64 (same as 65).

I did the same conversion about a year ago on a '67.
As mentioned...Simple, bolt in swap!