I modify the sexy ally proflow clevo pulleys and older xf non air con water pumps to give way more fan clearance.
But for a budget option slicing the threads off is no problem. You will find that the shaft material is very hard. Take it slow or cool the shaft with compressed air as you cut it to avoid damaging the pump seal.
Good rule of thumb I like to use is take the target HP and double it for the required carb CFM. Realistically, a healthy crossy would be in the 200-220 HP range without major work and lots of know-how. With this in mind, a carby between 400 and 500 would suit best.
There is a 4-barrel Edelbrock 500 that might fit the bill: