Fuel you pay on the installment plan. With the engine and tranny swap, you pay up front and wait for the savings to come back to you.
You'll have to ask yourself a few things:
- How much cash and time do you have for the swap?
- If you take both estimates and double them (not unheard of)... will it still be worth it?
- Will you hire it out or do it yourself? Changes the cost considerably.
- How long can you bus be down for engineering/ refitting?
- How long will you keep your coach?