I have a 351 windsor block with a 2 bolt main, is it possable to convert this to a 4 bolt main??

 

Yes it is possible. But you are not going to really get much more strength doing it. The stock windsors don't have that much "meat" in the lower webbing area. By the time you drill and tap for the outer bolts, you will be actually weaking the webbing. The best thing is to get a FRPP block or stud it with a main girdle.