1967-68 mustang

I suggest you consult the I6 to V8 swap sticky here:
http://forums.stangnet.com/showthread.php?t=705634

I think you'll find there are more answers to your questions (and more unasked questions as well) than you wish there were. Far too many variables to go down your list and provide single answers to those you asked. But I have a few rambling commments to try and help with....

Do the swap, even if you keep the I6 the V8 parts (steering suspension/brakes) are safer and will make your ride more enjoyable.

Do not underestimate the stock suspension, even with leaf springs and a worm-gear these cars can be made to handle quite well.

Buy parts as part of a package or kit to save money and ensure compatibility.

Don't go too far with wide tires and stiff springs or solid mounts unless you are building a track car or the roads you drive on are smooth as glass.

Nothing wrong with the one Vendor you named, but shop around, they all resell the same parts. Go with the company that has the best support staff (my opinion).

EDIT TO ADD: Relax in the comfort that this swap has been done tens of thousands of times. Talk to several vendors and discuss the entire project with detailed (but reasonable) expectations of your final goal. These guys want to help in addition to wanting to sell parts. Find the one you are most comfortable with after the discussion phase and buy everytihng from them, they'll hook you up with the right package according to your plans.