I am currently trying to get a Mark IXB ready for some vintage racing.

I have the rudder the boat came with which is very short and literally shaped like a chopped down scaffold pole.
I am in the process of reincarnating a bust up Turner fixed rudder from the early 90's that I found in my Dad's garage but it is an awful lot longer than the original.

Is it wise to use the newer longer and wider rudder for river sailing?
will it be an advantage or a disadvantage?

I welcome all opinions.

that would be scaffold plank, even I would know not to sail with a scaffold pole!

I'll be checking to make sure you're not sticking wings on it!

I thought as long as we called them fences they were allowed :-)

The rudder for my XII is 35 inches...I am launching the boat at Hampton this weekend for the 1st time..so if it is too short....

The answer is complex, if you go to the link below you'll find that there are whole groups of people discussing profles and sections.

But in simple terms, more surface area = more friction, short and fat better for low speeds, long and thin better for high speeds.

The Turner rudder in your dads garage will probably be fine for your needs.

The simple answer is to have both.
Passing Cloud has a long rudder which is essential for windy days like Salcombe this year and a short raked back rudder for river sailing. 
Windy races with a short raked back rudder normally end in a very early swim, you only need heel a bit too far and you can't steer.

I'm sure you Turner will be fine.....

That's exciting Mike, sorry I won't be there to see it as I'll be in Wales fishing for mackrel. The plan is to have mine ready for the Hampton Vintage regatta and silver tiller at the start of September.

Judging by general opinion (THANKS ALL) with the longer fatter rudder.

Nothing wrong with a Jon Turner rudder.  The shape is very similar to the Winder one without the little beak at the tip for catching the weed

Don't you have to be going backward to catch the weed on the beak :-)