LS TH400 help please

[LH Lead-Foot]

I am a little unclear on your project, but can figure out certain things to get your plans going.
#1 The TH400 will bolt on you "LS" engine, there is just no hole at the 2:00 O-Clock position. You will have to use the donor engines trans bolts and make sure you keep the locating dowel pins on both sides.
#2 The front drive shaft slip yoke works fine and is still used in 03 Tahoe.
#3 If the engine is a Vortec or truck engine and this is you "Dished Flex Plate" all you need is a $22 eBay adaptor that fits the nose of the torque convertor and the inside of the engines crankshaft to keep it aligned.
#4 Truck engines have an ugly intake but its design produces low end torque out the wazzoo and if you change the intake, yes, I would think about torque convertor change. The OEM TC, will have about an 1830 RPM stall. If the torque convertor is removed, look for a sticker. You can find internet answer on stall.
#5 Engine mounts are "Dirty Dingo" that everyone except me uses. Your mounts should be clam-shell with front to rear single bolt to bracket. They fit current mount stands and the correct dirty dingo plate allow for back & forth movement to clear headers, frame and tranny. Lot of automatics have to have the ears cut off for certain header applications.
#6 Headers have improved, but check out "Speed Engineering" as they have come out with a new header recently to fit the older "LS" C10 crowd that help get around the tranny.
#7 I have part #'s for VSS parts, but yes, you to do some work. The PCM requires a high resolution signal. Now, you will see a lot of specs on how many signals per mile you need, but it all come down to this.
(1) purchase a 40 tooth VSS wheel that goes onto the output shaft of your TH400. The ACDelco P/N 24202711 reluctor wheel. The old one come off with pushing down clip, tap and slide off back. New one is made of sintered iron and must be gently heated with a torch and have a flat cut end pipe that will go long enough to drive the new one on. It has no clip, just press-fit. Heating makes this easier to install as the can crack easy if not careful.
(2) VSS sensor is a phone call to "Monster Transmission" customer service can help find the right two wire sensor. You can order or get a wire connector from the bone yard. The sensor has two small metal bars that inside have a magnet between them. The wire core is wrapped where it produces an A/C signal get increase in voltage and frequency with speed. These wire go right into the PCM as a schematic will show VSS High, VSS Low. Either works. The wires coming from the VSS should be twisted then a round added. Place two wire into your bench vise, the other end in a drill, twist about 9 times per foot, but could it not critical. The ground simply goes to the frame with a clean spot, star washer and bolted down, then place it on the twisted pair all the way to the PCM. Have it long enough to exit at the PCM's harness, then go to the frame again. Home improvement stores sell metal tape that's 2 1/2" wide & about $8.00. Use electrical tape in a couple of places to hold everything together just a little, then wrap the entire harness with the foil tape the entire length. Start at a sharp angle to wrap sideways to make it easy. This helps keep RF induction or electrical noise out of the VSS signal to the PCM. Route next to the frame on the drivers side. Fabricate a metal "Heat Shield" if near the exhaust and use existing holes to mount as needed.
#8 The flex plate holes to torque convertor to not line up perfect. The OEM torque convertor mounting bosses are in a slightly smaller pattern. This is easy to fix with a "Rat Tail" file to move the hole about 3/16" inward of smaller.
Hope this helps. Retired ASE Master Tech / master tech since 1978