fishtale,
First of all, attaching a piece of lead-core to the end of the floating tip of a spey line is not a good idea and it will badly overload the rod as well. Second, either cut the MidSpey 15 1/5 feet from the tip and install loops or buy the MidSpey with tips that comes with loops installed by the factory and several different sink rate sink tips.
Third, although you could cast 350 gr sinking shooting heads with your 2-hander, it is not the recommended route to go. If you really are sold on using 350 gr sinking heads on your 2-hander, you will have to either cut the MidSpey 25 feet from the tip and install loops, or use the MidSpey with tips and remove both tip 1 and tip 2, then loop your 350 gr sinking head to what is left. This will give you a line that is about 40 feet long. And that is a very short shooting head spey line.
Fourth, why bother with a 350 gr sinking head or sink tip in the first place with your 2-hander? The rod will cast any 9 weight 15 foot sink tip and since sink tips are available up the a sink rate of 8 (about 8 inches/second, in other words, really fast sinking), it is not necessary to use a 350 gr tip on the 2-hander.