If you want to actually make spey casts and throw modest sizes flies, an Airflo Delta 8/9 multi tip line would be an excellent choice as the 54 foot head carries enough mass near the sink tip junction to turnover the tip and fly well. A mid or long belly line will require more skill as you will have to create more line speed, be more precise in your casting path and acceleration points and it will be tougher to lift tip and fly from the water. The floating tip section will also allow nice dry line casting, and the Delta taper is a really nice line to work with in general.

If you really want to throw heavy flies and tips like straight lengths of T-14 the Skagit system is the way to go. Both Rio and Airflo make great off the shelf skagit head systems now so no splicing and tinkering would be required. Skagit casting is not the same as traditional spey casting or even underhand casting. This water load cast is a method of getting a very heavy, short head up and off the water. The heavy head is the best way to carry the heavy tips and flies, and is very effective, but if you want to be casting those long pretty loops you may not be happy with it.