I am not an expert on this, but I have two cars with the switch pitch 300 and one of my cars had a 300 non-switch pitch when I purchased it. With-out the wire going to the kick-down solenoid (inside the trans) no signal will be given to the transmission and it won't downshift when you floor it to pass (passing gear). With-out the wire going to the switch pitch solenoid (inside the trans) no signal will be given to change the torque converter ratio, so at a stop sign more braking is needed (probably not a show stopper), and no increase in torque with an increase in throttle position; 1/2 to 3/4 throttle. The car will shift from 1st to 2nd (vacuum line must be connected).
I should say that if the 350 engine is used, I would go with the 350 transmission(pairing the original set-up). It has been my experience that the 300 switch pitch is a nice set-up if you understand how it operates. The 350 engine with the 350 three speed transmission should put a smile on your face as well. Messing with the 300 throttle linkage on a 350 engine could be a pain in the butt.
Obviously the engine compartment is going to be non-stock, at that point it is ultimately what do you want to drive and when do you want to hit the road!!!!
Sorry if I made things too complicated, but the Switch pitch can be a little confusing (especially for me).
Good luck, Randy