Prop turning fast when engine started

Happy Easter everyone. I bought a project boat last fall. It's a 79 Ski Nautique with a Ford 351 with a Velvet Drive that is in need of some love. I've been working on it all winter and was finally warm enough last week to try to start it. I never saw the boat run before buying it, but the deal was too good to pass up. I was able to get the boat to start, but the prop is running full boar without any throttle. This happens whether the throttle pin is pulled out or not. Ive tried to step on the shaft to see how much power is getting to it, but that didn't slow it down. I've also tried to disconnect the throttle cable from he transmission and that did nothing as well. I have read a lot of people asking about their props running a rotation or two a second, but this is significantly more than that. Any thoughts on what may be causing this or how it can be fixed?

Did you try wiggling the shift lever forward and back to see if your tranny is just adjusted wrong or maybe the detents in the tranny are not working properly or stuck.
Do Forward and Reverse function? I know you have forward movement but what happens when you actually shift into forward and later into reverse?   Just curious if the tranny actually works in reverse. Is the fluid clean?   You might be into a rebuild but rule out simple stuff first.   Maybe activating reverse will loosen up the clutches in neutral.

The shaft starts to slow down because you're releasing the pressure on the forward clutch piston and applying pressure to the reverse piston as you're shifting from Forward thru Neutral to Reverse.

If you go from Reverse to Neutral, does the shaft start spinning Forward right away or stay stopped for a while.

Myself, I never try to stop the shaft with body parts, not even somebody else's

Try a 2X4 wedged under the coupling to see how much effort it takes to stop the shaft in Neutral.

Since the boat shifts OK, you might want to drive the boat around some and get the fluid warmed up to normal temperatures and see if the condition improves with some running time.

You've got nothing to lose by giving it some driving exercise but like Pete said then you're into transmission work if it's still spinning so much that it's hard to drive/control.

Some people have a dirty neutral and have lived with it for a long long time.