Slant six dies in gear

My guess is that your transfer-slot is as good as closed. Increase your rpm 150 rpm then retard the timing until the rpm drops to about 600, or the timing is no less than 5*. reset your mixture screws as may be required.
If that don't get it, reset you valve lash .003 looser on the intake, .006 looser on the exhaust, to a maximum of .013int/.023ex; hot to warm, but not cold.
If that don't get it, jack the rear wheels off the ground, put the trans in Neutral, and try again. If the rear wheels turn, start dragging the brakes. If the engine slows considerably or stalls, BadaBoom! the trans/TC has to come down.

This assumes the following:
1) as to the air intake;
the PCV system is working properly
The brake booster system is not sucking air
the intake has NO vacuum leaks
anything connected to the intake is functioning correctly
the valves seal nicely, and are correctly lashed

2) As to the fuel system:
The Wet fuel-level is correct
the fuel pressure is 4.5 psi or less
The choke is OFF,
the mixture screws are in range
the Air-bleeds are clear
the fuel is fresh and NOT contaminated

3) As to other:
The VA is plumbed to the sparkport
the engine is up to temp
the convertor is functioning correctly
the trans clutches are fully released
The pistons/rings are not tight
the A/C system is not overcharged, and the clutch is releasing,

Ok so that's all I can think of.