Need help i.d.'ing transmission / spindle count

[Uwe Blab]

I have the casting numbers from my truck's gear box (3925647) and tailhousing (3880042). The truck is a 1970 Chevy LWB fleetside, straight 6, 3-speed on the column. Is there any way to identify the spindle count and diameters for the input and output shafts from these numbers? Also, can I verify the overall transmission length? I put a tap measure to it and it looks like 21-1/2", but if there's a way to verify I'd appreciate it.

Thanks for any help.

Adam