I'm putting on a 250 blower I have 2 crossover pipes to choose from one with a bypass one without, the bypass one is smaller dia. then the other but is a thermostat necessary with a blower so I'm asking which crossover is best to use, with or without bypass

You really need to use the bypass if you want to be able to control your water pressure and temps with any degree of accuracy. You can control temps somewhat by using various size restrictor plates. If you operate in varied water temps you will need to vary the size of the restrictor. Without the T-stat you will may get lucky and find the size that gives you proper water temp.

By using a T-stat and a dump valve of some sort you can control your water temp and pressure. Without a T-stat your oil temps may be too cold or too hot as the restrictor regulates the flow of water through your oil cooler. Do a search on dump valve and water pressure.

I would also recommend a very good oil cooler and a T-stat for your oil. I am using the large Eddie Marine with a Perma Cool T-stat for the oil. Home built dump valve using parts from Mc Master Carr and some varied fittings on my B&M 250 setup. I have pretty steady oil temps from 210 to 240, water temp stays between 120 and 130 with water pressure 5 or so at idle and 15 to 20 at WOT regardless of ambient or water temps. It will take a little work to get it right but water temp, water pressure and oil temps are critical for a blower motor.

Hey BadDog,
what thermostat are you using?
Thanks,
Dave