Six,
Another possibility is the NV-1500 and NV-3500. They will bolt up to your 66 inline or V/8. They are from 90's fullsize Chevy trucks. They are like an automatic as the they are bellhousing inclusive. Outstanding transmissions! Have one bolted up to a 327 in our 50 Sedan Delivery. These are strong 5spd's.
Only problem on your truck is the rear x-member at the back of the engine interferes with the belhousing of the transmission. If you have the skill and you can make it work.
Otherwise the T-5 is your best bet and the easiest to install since you retain the original bellhousing and mounts! We converted a 60 1/2 ton with the T-5 and it is running 400 HP!
47 Fleetmaster, Hotrod 6, 5 Speed
50 Sedan Delivery, 327, 5 Speed
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