By chance does a 260z dashboard fit in a 240z? Along with seats?

Seats will fit just fine with no problems.

Dash will fit but there are some minor differences that will have to be addressed. Such as some of the wiring connectors for the gauges and harness may not be compatible. The easiest way might be to re-use the 240 gauges and just use the 260 dashboard shell. You may run into a few little things to be worked out but it shouldn't be too difficult to make it work OK.