drivetrain slack

[brycer1968]

After three years in my driveway, hundreds of hours and some blood, sweat and tears- the M725 is finally drivable again. Thats been a great achievement for me, but I'm observing something when I drive that feels and sounds like excessive driveline slack and was wondering if anyone has any experience with this and might be able to help ID the likely causes and fixes.

My build:

• Stock 350 SBC with Fitech fuel injection, SM465 four speed connected via a very short custom Spicer slip-jointed shaft to the stock NP200 that drives the rear axle through a rear driveline relocation kit.
• SBC front motor mounts are new rubber Anchor mounts and the same for the rear transmission mount. Factory rubber TC mounts are old, but tightened down with new hardware. SBC motor mounts don't appear to have any free play or issues that I can see.
• Both stock driveline U-joints are also new and both stock axles have been regeard with 4.56 gears by the local Six States/Spicer place. They said that the stock spider gears were cleaned and reused.
• The TC and SM465 both seem to have some wear, but seem to work okay and I see that the TC has gotten hot in the past. Both TC and SM465 came to me with pretty major leaks and had been run low on oil in the past. (The SM465 is on my winter rebuild list and the NP200 is going to be swapped for a divorced rebuilt NP205. The clutch and TO bearing will get changed too.)
• All transmission and transfer case fluids were changed and so were all the shaft seals that were accessible from below the vehicle -and the TC and SM465 are both holding oil now.
• I didn't touch the clutch other than adjusting the pedal mechanical linkage free play. Clutch holds okay and is much smoother than I thought it would be.
• All leaf springs were cleaned and painted and spring eyes/bushings replaced with new.
• Drum brakes are also all new parts.

The clunk:
In 2wd high, when I let off the gas and then push the clutch pedal down, there is a clunk or metallic thud that is louder between 2-3 shift than the 3-4 shift. It can be felt through the floor and heard loudly in the cab. I can lessen the clunk somewhat by being more delicate/slower with the clutch, but its still there. Also, the exact same noise can be duplicated in any gear by getting off the gas and than mashing back down on it quickly.

I know this is difficult to diagnose via the forum, but if you've read this far, I'd appreciate any thoughts/theories or ideas on what driveline components may be responsible for this noise, any tests I should conduct to narrow the "suspect list" and if the TC swap and SM465 rebuild is likely to fix this or if it's likely something else?

Thanks,
Bryce