You can use the 351w exhaust manifolds, but a 351-specific z-bar will be required. New H-pipe will be required.

Does it have ac or power steering? If not, all you need is the 1969 351w (one year only) cast iron alternator bracket. The two stamped steel brackets you have now will fit.
If it has ac and ps, you need the 1969 compressor brackets

Everything else bolts in the same as 289..

What is a tristar?

I would just rebuild the 351 you already have.  The problem with rebuilders is you get whatever parts they get the best deal on, not necessarily the best parts for your goals.  Choices like cams: roller vs. flat tappet, etc. are made for you.  You also have to know how they h.ad the engine configured on the. dyno to make the stated HP, or you likely won't get there.  I would guess that any engine they dynod was run with. headers, if nothing. else due to the simplicity of connecting. it to the dyno.  BUT, I've built a lot of engines, so that task isn't daunting. for me.  If its outside your wheelhouse then a target engine is likely a better choice. 

Of course I chose to swap in a 5 speed before the better engine I'm building, so that would be my preference.  I'm sure I'll love the additional power when I get it, but the ability to cruise down the highway trumped that for me.  That's all about how you plan to use the car too though. 

Were putting a 351w in my son's 67 Mustang.   Were using a 95 F150 Front dress on it.  Here's a pic with the serpentine belt system that will have on it.