I having problems with my 1952 M38A1

[Deadguyllc]

the transmission, transfer case, and front and rear axles are all geared like a 1952 M38A1. I seem to recall, when I first got the vehicle, that it was hard to get it to about 55 without the engine screaming. I installed an overdrive, with a rebuilt T90 transmission and T18 transfer case from Novak. I then had to get a shifter lever for the overdrive from Herm. I thought the overdrive was manually engaged while I waited for the shift lever, and the vehicle could easily do 55-60 miles an hour. Shifter lever installed, I move the lever about, and it won't stay forward, with the overdrive engaged. I push it in, and a minute later it pops back out, into neutral, and I have to pull the lever back, into direct. I drives fine in direct, and does the higher speeds like the overdrive is engaged. I had the overdrive pulled at a friend of my friend's shop, and he tried removing and reinstalling it 3 times. Still popped out of gear. I pulled the whole thing and sent it back to Novak. They say nothing is wrong with it, and shipped it back. It hasn't arrived yet, but if they did almost nothing, I have no idea what is wrong. I talked to Herm, the overdrive guy, and he says I might need shims behind the barrel connector, to push it forward so it makes a more solid engagement with the overdrive. Any opinions on this?