I am running a 280z booster in my 72 240 and it needs replacing. Is there an upside or downside to using this slightly larger booster on a stock 240. If its better I'll get another 280 booster, if not I'll go back to standard. Oh, Im also using 280z mastercylinder.

No huge benefit as far as I can see, unless you were going to use the later 15/16 master cylinder off a 280ZX. But, without knowing what type of brakes you are running, hard to judge any benefit. For stock brakes, I'd say it would be a waste of money to go larger on the booster or the master at all.

For 4 piston calipers in front and rear discs, you'd need the larger master but the stock booster should be OK with that set up.

Truthfully, I don't know that there is that much difference between the 240 and 280 booster at all, so any benefit would be incremental at best.

Possibly cost, but I am not sure on that one...

I need to replace the booster on my '71 series 1 and I have a later setup on another car...

Was the swap hard?

I have considered the swap but have not completed it...

Any advice?

The swap was fairly easy, I just had to drill 2 holes in the firewll for the wider bolt configuration on the 280 booster. Just make sure you line up the plunger to the pedal arm(not rocket science).