What should be the siamese size be roughly? I am going to buy the slp runners to match it up. And when i do that should i bore the bottom plenum or just buy a new bigger one like the edelbrock.

 

Hard to say. Not sure what you are asking. Are you trying to clean up the siamesed part? Or are you asking how you should make the transition from the plenum to the runners?

I opened the runners to the plenum so they were shaped like a wide '8' and then enlarged the siamese portion to add more plenum area. then I sharpened the material where the runners split without really taking it that deep into the runners. Keep in mind, the more you siamese the runners, the more you are adding plenum area, but the more you are bound to loose the LTR effect (low end torque)... it all depends on your setup and how you port it.

I've seen some guys make a large oval out of the runners instead of an '8' to maximize plenum area, then smooth out the transition into the individual runner tubes.

As far as the rest of it, I would probably just port match the runners to the base and smooth it out with tootsie rolls and call it a day, unless you are looking for maximum power. If you are looking for more performance,that I would at least get an accel base or an edelbrock and port match it to that. I wouldnt recommend siamesing the base unless you absolutely know what you are doing and what you want.