Bearing spacers were for the 388 crowd, they use a 350 crank in a 400 block as the mains on a 350 are smaller when putting a 350 crank in the 400 block. As for the 1st. 383, I read about it in a Car Craft like in very early 80's. They took a 400 crank turned down the mains to a 350 size but they also had to clearance the block for the 400 throws at the time and clearance the rods for a std. base circle cam.