Can you weld and do you have a mig/tig that can weld up to 3/16" steel? If not, the bolt on kit will probably save you money vs. paying a shop to weld in the crossmember. I haven't installed one on a mustang but a frame (unibody or full) is still a frame. I bought a helix kit for my 57 chevy pickup and other than measuring 37 times and cutting once, it is pretty straight forward. It looks like a lot bigger job than it really is. The only hard part is getting the crossmember welded in place.
Also, these kits are pretty generic. They all say vehicle specific but they are all designed around a mustang II platform. The crossmember for a 69 mustang is the same (before YOU cut it for the frame) as the one they have for a 32 ford. If you can weld and know what you want out of an IFS kit, you can probably save yourself $500+ by ordering a crossmember and putting the parts together for yourself.