I don't remember right off what the first year the Detroit Diesel 4-53 was offered in a truck? But I think it was the next body style?
Either way, I think one would fit reasonably well in a 66.
Not knowing what transmission you have now, If it has an SAE clutch housing then it can be used behind the Detroit. And even if it does not there is still a chance that it could fit with off the shelf parts.
But realistically you will want an OD with whatever diesel you choose, assuming it has enough HP to travel at a reasonable speed. If not an OD transmission, then faster gears in the rear axle.
Diesels generally have a lower operating RPM range than a similar gas engine, so to get the same speed and driving characteristics you need faster gears when you swap to the diesel engine.
The cost and time involved in a diesel engine swap is almost never worth the savings in fuel. So if you want a diesel engine do it because you will enjoy it, not because you think you will save money, you won't.
Grigg
