406TD engine into 306?

Yeah they use same engines in each car other than the v6 but it will be fun and hard work I would imagine

If it's a normal like for like ~90bhp, 1.9l XUD9T swap then it should be a straightforward operation. There may be differences such as placing of the intercooler and suchlike (306's the intercooler is on top of the engine - dunno about 406's), best to check. Peugeot did definately use the same

A 2.0HDi for 1.9TD swap isnt as simple, in fact probably more trouble than its worth. HDi engines have electronic jiggery pokery, electric fuel pumps and drive by wire throttles and things.

Perhaps make sure that the donor and recipient cars are of a similar age. Hell, if the engines look the same its probably gonna be OK. HDi's have a dirty big bit of plastic attatched to the top usually, TD's have an intercooler.

Warmest Regards Douglas