TH400 speedo driven gear issues

Hey y'all,
I'm dealing with a TH400 for the first time in my personal gearhead journey and am having issues trying to adjust the speedometer driven gear to get the right value on the speedo gauge.
I swapped out the original 40-tooth gear for a 36 and thought it odd that the new driven gear didn't seem to touch the drive gear after installing it. But figured things would change once the truck got moving. But the needle didn't budge (just laid flat on the gauge).
Then I noticed the numbers cast into the housing. So I found a different housing with the right numbers thinking it would have a longer snout to push the driven gear into the drive gear. But it looks to my untrained eye to be exactly the same.
I've yet to install this new housing, but figured I'd ask the audience what I'm missing before I try (and fail?) a second time.
Am I missing a bushing or something in this combo to get the driven gear to mesh with the drive gear?
I did notice you can't fit the 40-tooth gear into the 36-39 housing due to it having a very, very slight step in the shaft. But there's no step in my eBay-sourced 36-tooth gear.

What drive gear is in transmission I bet it has the metal gear

It's plastic as well.

Follow-up:

I plugged the 36-tooth driven gear into the 36-39 housing and plugged it into the tail shaft. Seems to work fine.

Problem is: I'm still 5mph too slow on the gauge. (Pretty much exactly the same as it was with the original 40-tooth driven gear.)

Changing the tooth count is going to change the ratio. Not the calibration. If you have the wrong gear your speed will be off by a percentage of MPH. Being 5 MPH off all the time means the speedometer head needs to be recalibrated. Usually they're correct if the needle is pointed at the little calibration mark on the face of the gauge when the speedo is at rest.

so the original drive gear on the 400 was a 40 anybody out there know what the original drive gear was on the 350?

Blue Ox said:

Changing the tooth count is going to change the ratio. Not the calibration. If you have the wrong gear your speed will be off by a percentage of MPH. Being 5 MPH off all the time means the speedometer head needs to be recalibrated.

Click to expand...

Several years back, I was able to plug in a different-count driven gear on my TH200 4R in my 68 Biscayne and it got my speedo a good bit closer to accurate. Never touched the speedo unit.
I figured I could do the same here. Bummer.
I have this convertor/adaptor that came on the truck when I first got it. It spins on both ends just fine (input turns the output as normal), but when I plug it in between the driven gear housing and the cable, my speedo needle just lays flat as I go down the road.

Yea i Changed the rear axle gear on mine from 2.73 to 3.73 and it threw my speedometer off by 20. mine has the 40 tooth and that adapter. so now trying to figure out what tooth i need the calculator i used said 54 haha.