I agree with NWC to go see a prop specialist.
There are websites, such as Mercury , that have prop charts, but the situation here is that your setup is not stock...
The first thing I would do is get rid of that OMC and find yourself a good used Bravo. Parts are getting hard to find for OMC and when you do, they are dear! I had an OMC on my 210 many years ago and I had to change the ring gear on the drive every single year! (Not a good system) Universal joints are much stronger and a better drive system, especially for the torque that a big block will produce.
Back to the issue of the prop..... since you have a big block with lots of torque, I would recomend starting out with a 19 pitch and a maybe going to a max of 23-25 on trials. See what your rpm is and try to get the rpm to spin at WOT about 5300-5500rpm. Those big blocks should take the punishment with ease as mine spin at 5600 no problem.
A four blade would probably work well with that size boat but it would go good with a three blade too.
Another factor to consider is the X-dimension. If the drive is shallow enough you could even consider getting a "clever" althogh when I tried a set of those on my 260 they had no top end, but I think it was because my drive were too deep???
Hope that helps?