Swing blade gearbox.

[Grower]

Hello, I am looking at building a swing blade mill but want to ask about gearboxes. I am attaching a photo of one I have on hand (I have 3 but want to focus on this one), it is off a piece of agricultural machinery that was scuttled for parts some time ago. The input shaft runs at 540rpm (a standard PTO speed) and the box is rated approx 35hp (it could be 40hp I do not have the owners manual on hand but the machine was used for a number of years on the farm by me), the input shaft runs at a ratio of 4:3 (3 being the output), I want to know if it can be run in reverse at 2400 rpm input speed via a diesel engine to get an output speed of 3,000 rpm to run the blade at. What would be the consequences, if any?

[TRTermite]

The blade speed is relevant/proportionate to the blade diameter. 10,000 rpm being EXTREME production Max. 
Reversing rotation or input "STUFF" I cannot help you with. 

[maple flats]

You really want to go by surface feet per minute, not rpm. A 7" blade will run at 3,000, rpm while a 20" blade might only be 700 rpm. Those numbers may be all wet but they are just for reference. My Peterson  had a 21.?? blade diameter and i'm fairly sure it ran at under 1000 rpm, with a 20 hp Honda on it.
If the diesel engine was turning the blade at 3,000 rpm you might not be able to keep teeth from flying off from centrifical force. I think you need to slow it down a lot, others  will likely chime in, if I'm off base. Here's a chart i got from Vermont American for maxinmum RPM by diameter of the blade.   

Home » Circular Saw Blades » Circular Saw Blade Speed Chart

Circular Saw Blade Speed Chart
Recommended Operating Speeds for Circular Saw Blades
For extended blade life and quality of cut, saw blades should run between 10,000 and 18,000 SFPM (surface feet per minute). Never exceed 18,000 SFPM!  To calculate SFPM: (1) Multiply the diameter of the blade times Pi (3.1416) to get the circumference, then multiply circumference by RPM to get inches per minute and (2) divide the inches per minute calculation (from step 1) by 12 to get the feet per minute. Best results on most materials would be about 15,000 SFPM.

Circular-Saw-Blade-Speed-Chart