Dowels are very much needed. They are for proper alignment. If your starter makes strange noises that is the first sign. I know someone who broke 3 starters before he realized the importance of dowels. Thing is he was using them. But they were installed in the tranny and not the engine. When they are in the tranny they go in too deep and they wont reach the engine. If dowels arent present the face of tranny cant shift around on the engine. doesnt matter how tight you think the bolts are. The clutch disc will feel major pain as the flywheel will spin it off center. very likely to cause undue chatter which would kill the springs quickly.