Stumped - Van keeps stalling

My Van: 69' G20 3/4ton 307/th350

The van starts & idles just fine but as soon as I put it in gear it stalls the motor. It will start right back up as if nothing is wrong.

I can get to stay running in gear using the choke. Once the van is driving I can return the choke to off & the van seems to drive fine (max speed so far 50mph) until I come to a slow crawl or stop & then the motor dies but will start right back up without hesitation. What kind of problem should I be looking for.

I've checked or replaced:
carburetor
cap, rotor, condenser & points
plugs & wires
set timing

thanks for any help.

Idle speed in Park should be ~900 RPM. About a 200 RPM drop in gear. With a razor sharp tune, you can lower those numbers 1-200 RPM. However, I usually don't on these vans due to slow fan speed and likely tired engine which means low oil pressure. So let's just go with the first specs.
Idle mixture is important at idle, it's the only circuit feeding fuel to the motor.
Also look at initial timing and if the vacuum advance is connected to manifold vacuum or ported vacuum. I prefer around 5-10 BTDC initial timing with the vacuum advance disconnected. If connected to manifold vacuum, the drop in RPM from trans drag plus the drop in vacuum can often cause the engine to die. It takes a bit more finesse to get these to be happy. If it's connected to ported vacuum, it should not be getting vacuum at idle.
Also check for vacuum leaks. You didn't mention idle quality, but if it's rough, one or more cylinders could be lean at idle, causing them to not pull their fair share.

I have had some Chevys, mainly PG equipped, that pulled down 3-400 RPM at idle. It's a function of a tight torque converter. Then you have to carefully balance idle in gear with idle in Park so it doesn't diesel.
So, set the points to 30 degrees, timing at 5-10 BTDC, set idle at 900 in Park, adjust the carb for best smooth idle, make sure there are no leaks and then drop it into gear. If it dies, raise the idle speed 100 RPM and readjust the carb. Once it idles in gear, re-adjust the idle mix for best idle. You can use a vacuum gauge or a tach until you develop the 'feel'.
VAcuum should be 16-18" at idle in Park, 13-15" in gear, although a tight converter can pull it down more.

THANK YOU both !!!!

Keeps running when using the choke=??: that is specific to the carn =NOT =getting enough fuel. Cutting down airflow9choke) is typical of an internel fuel flow problem. Take carb apart & look to see if the jets are coroded or some trash has entered float bowl & has some part of jets or areas inside clogged up?

Also, don't count on the timing being right per the markers on the lower pulley--Harmonic Balancer. The outside rings can spin around as the 40 year old rubber contracts. Might check it against TDC of #1 to see if close. Also, if you have really bad compression in a number of the cylinders they can idle okay yet have no power under load. Hopefully that's not the case as getting compression back is pretty much a rebuild. Good luck and please tell us what was the fix.