I changed from TH350 to SM465..once..
My 72 K5 I had years ago with a Th350 would not go into any forward gears for at least 5 minites,longer in cold weather!--once it shifted in when the motor got up to temp,I could drive it all day,and it worked fine until the next "cold start" after sitting all night..this led to me being late for work a good many times--finally,the poop hit the fan,and my boss told me if I didnt start getting to work on time--I could look for work elsewhere!--
So out went the TH350 in favor of an SM465 setup out of a wrecked K5 like mine that WAS mint,until the owner slid it broadside into a fire hydrant.drunk out of his mind on easter sunday--I used to drool over his truck every morning,it was always parked at a "Firestone" tire place next to mcdonalds where I ate breakfast often..now it was totalled!--but I got a lot of good parts for 300 bucks,like the entire 4 speed setup,everything to convert it,power brake booster,peadls,etc...
But after a week of 6pm to midnite work sessions,I finally got it all in and driveable--first thing I noticed was the "growling" noise coming from the coupler that joins the tranny and transfer case--I drove it several months,and eventually pulled the T-case out,and found the coupler,and the splines on the tranny's output shaft were worn pretty bad--had no cash to waste,so I threw in a brand new coupler with a ton of green "locktite" bearing and sleeve retainer compound in it,and hoped for the best....
I drove it about another year,it was starting to get noisy again--I kicked myself for not just having the TH350 rebuilt and re-installing it...the 4 speeds gear ratio spread sucked--1st gear is for climbing walls or pulling a heavy trailer at a crawl,useless in city driving..2nd gear is like first in most "normal" tranny's,a bit on the "low" side..the jump to 3rd was a big one!--you had better redline it in 2nd gear before shifting into 3rd,or it will bog and fall flat on its face--you could wind it up to 65 in 3rd though,good for "passing gear" on the highway--4th was like overdrive,the K5 had 3.08 gears,too high for a manual tranny in most cases...
I wasn't real happy with the manual tranny,especially when driving in city traffic--the 12 inch 3 finger clutch was murder on my left leg,and it wasn;t long before 2 of them had been installed,due too defective pressure plates,and I eventually put a "centerforce" 11 inch clutch(diaphram style) in its place--then the issues with the "coupler sleeve" started in..
I've had fairly good luck with TH350's,only the on in my 74 K20 had finally lost reverse after 15 years of beatings with a 454 and a 400SB and lots of abuse snowplowing with it...other than the one in my K5 that had the "delay" in forward gears,all the others I've had held up good,even behind 454's...I had a few TH400's in cars that were pretty rugged too,but they all can suck in one way or another I guess...I prefer the automatics until they fail,then I wish it was a standard!--at least when you let the clutch out on a manual,you can be 99% sure its going to move!--and putting a new clutch in is easy compared to tearing down an automatic--I doubt I could ever rebuild one successfully...
I wished my K5 had a tranny like a close ratio Muncie 4 speed like the vette's and camaro's used--Advance Adapters made an adapter to put them in a 4x4,I think that would be a good tranny,because the gears arent so far apart in ratios..
.I still dont know why GM hasn't made a beefy,yet streetable multi-speed streetable tranny with close ratios--maybe the newer trucks 5 speeds and 6 speeds are better,I have no experience with them--but the SM465 tranny is a good one for a dump truck or bus,but not exactly street friendly--many other friends had troubles with them popping out of gear due to broken snap rings,and the tailshaft splines wearing out...seems like no matter WHAT tranny you have,they all have their downfalls...
and this is after driving 1000+ miles w/o clutch in a 465 setup 
But that's a driver prefference as I despise heavy traffic anyway (read I am 200% country boy 
