I did my mains in place and I did drop the crank some. Mine is a 216 with the dowels. I removed the front cover to give room for the drop and also observe the timing gear. If you don't have the dowels you should be able to drop the cranks very slightly and change the bearings. I would rotate the upper shell so that the tab comes out first. The thrust bearing is probably the hardest to change because of the sides.
If you check the clearance and it is not excessive, I would leave them in place. You can get an idea of ahwt kind of shape they are in by looking at the bottom halves.
