Two rules of thumb on props is when changing from a 3 blade to a 4 blade you go down one prop size in pitch which usually means 2 inches of pitch. Second every inch of pitch will usually change your rpm's by 200.
I would think your boat would work best with a 19p 3 blade. Going from a 17p 3 blade to a 19p 4 blade you may see a rpm drop of 4-500 or more. May as well try it at this point. You got nothing to loose. Max rpm range for a 260hp merc is 44-4600.
http://www.mercurymarine.com/propellers/prop-selector/#

I would also think your speedometer is wrong for a 17P prop at 4600rpm.Probably more like mid 40's.
http://www.marksmarineinc.com/prop_calc.aspx

that 53 mph was not dirived from the "dreamometer" but was actually a GPS reading...

and power is from a 305 small block, not a 350. I suppose I will just need to find out how thigs work in 4 months or so when winter is over here in CT....providing it ever begins??? I am wondering if I should just put the snowmobiles away, and start getting the boat ready?

Ok, kinda back to square one.

First of all, hmmm, yes a bit of a mistake. No the Trophy+ will be the wrong prop. Not trying to be a stick in the mud, but it is what it is. Still, bolt it on and see what happens, you have it anyway.
This is where it gets interesting and to some, maybe a bit confusing.

Story time.

The Vensura is actually the original Trophy before there was a Trophy+. Then it became the Offshore and now it's known as the Vensura, still same prop. Blades are a bit different on the +, and the barrel is smaller. This is done for the outboards to vent exhaust over the blades when accelerating. What your speed has been with the alu prop is not that important, your rpms are. Basically the same thing goes, but what you then need is a 17". Again the LaserII or Vensura are the props to look for. Sell the Trophy+ and tell the guy who sold it to you, that they are NOT the same props, at all.

For all around performance on your boat, the 4 blade will work the best.

Oh well... I wonder if he wants it back? He wanted 250 and sold it for $200.... if he takes it back at least he will have the opportinity to collect on the $250. I will even c if he wants to keeps a few bucks for his troubles... he did explain the venting that u did A.O. and con.tnued with the Alpha I is the same basic lower end and would work the same as if it were on an outboard... at least he.was trying to be honest with me...

What would u imagine would happen when the exhaust vented over my prop on my I/o?

Blow out when turning, when pulling and when accelerating. Lack of carrying ablillity, over reving, poor trim response and efficiency. possible cavitation burns. Did I mention blow out.

Hahaha...ok, I am convinced... its gotta go! Anyone wanna buy a prop? Hahaha

You said in your first post it wasn't the 5.0ltr.. 305 is the 5.0ltr. 230hp. If your confident that your GPS is correct then your tach is probably wrong. 53 mph with a 17p prop would actually be about 5100rpm's. A 19p, or 2 more inches of pitch would bring the rpm's down by about 400 which would put you closer to the correct recommended max rpm.

The flash arrester reads 5.7 ltr. Which I know is wrong.... and must be off a 350ci motor.

The valve covers read "228" (whatever that is???)

and the sticker on the side of the head reads "305"...

I guess I thought a 5.0ltr motor was a 350.

I am sorry for the confusion. God I hate being new to something...!!! Thank you to everyone for their patience...
Be sure to tell all your friends that have outboards that I have a nice SS prop for sale for cheap.

Try posting prop here, may help in selling:

http://www.bassboatcentral.com/props4sale2.htm

I will definitely do that. Thank you!