If you're pulling the dipstick while it's running I'm 99.9% sure it's just oil slinging off of the clutch. I would probably save from doing that anymore unless you want a big mess. Not sure how it could be building pressure in the transmission area unless you've got a right-side crank seal leaking or somehow you've got a leak in the case halves themselves letting pressure get out of the crankcase. That's the only way it could be building pressure in that specific area, plus you have two vents, one right under the rear engine mount and one under the impeller cover on the clutch side so there shouldn't be any considerable pressure building up in there. My guess is that you could have an air bubble somewhere. Jack up the front end a couple of feet in the air or close to it and see if your coolant goes down. If not that then I'd look at the radiator itself. It's a pretty self explanatory system so if it's put together correctly and has no blockages or leaks then it has to work.
Another thing, is it spitting out of the overflow all of the time or just when it gets hot? If it's all of the time then of course that would mean that you are getting compression into the cooling system and you'll need to tear it back down.