can someone tell me if i can take a standard rotating merc 260 and just remove the timing chain and switch it to timing gears .or are there other mods to be done

Cam, oil pump, water pump all have to be changed I believe.

pistons need to be rotated 180* to correct the thrust offset

notch at the top of the piston that faces the front needs to face the rear

first you need a RR cam to go with that gear /timing chain change. If not your standard cam will open the exhaust first and then the intake valves. The main crank and harmonic balancer seals have to be changed or instead of holding oil in they will leak oil out. If the crank has been used as a standard rotation it will at least need to be polished or honed so the microscopic scales lay the right way with new bearings.
Most RR motors I have dealt with pretty much the only thing that spins backwards is the crank. Since the crank spins backwards you use the timing gears instead on the two sprockets and chain to spin the cam back the normal way. The cam spinning the normal way means the oil pump, distributor and most everything else are all normal off the shelf parts.

i think you will have big problems if you try this.the valve relief will be no where near the valves and contact will happen and destroy the engine.

I pulled a 327 Chevrolet apart that was reverse rotation and that is exactly how they did it, notches to the rear. The pistons had 4 valve reliefs in them though.

Why would you want reverse rotation?

Rear main seal is different from std to reverse rotation. sorry didn't see this was already addressed

Put a Bravo behind a normal rotation. hah !

or a Volvo Penta