I installed a new top on my '76 last year. Got the cheapest one from LBC Co. with a zip-out rear window. (It was on sale.) This was the first time I'd attempted anything like it, and it was easy peasy.
<span style="color: #CC0000">Voice of Experience:</span> Measure twice and cut once, take your time, if it's vinyl do it on a hot summer day in the shade, put it on hot and don't stretch it too tight. It's not a difficult thing to do at all. Don't go to extremes trying to get it drum-tight; that'll cause trouble later when it gets cold. Just put it on snug enough so that the wrinkles are pulled out.
You can carve the Tenax holes with an exacto knife if you're careful, and install the Tenaxes with snap-ring pliers. The snaps and snap-anvils can be bought at Wal-Mart.
Use chalk to mark the top's center line. Use spray adhesive to position the top at the front rail before riveting it down.
<span style="color: #CC0000">Fun stuff to try:</span> Instead of screwing the retaining bar down through the back edge of the top, screw the retaining bar right to the car itself...Then install snaps on the back edge, and snap the top down to the bar. You can then fold the top down without putting any stress at all on the windows. I did it this way, and it works a treat. Gerard has kindly put my tutorial on "folding a zip-window convertible top" up
on his website and you can get a good look at what I mean.
<span style="color: #CC0000">Stuff I would do differently:</span> I'd try installing the top to the header rail with sheet metal screws instead of rivets. It'd be a lot less trouble when the time comes to replace the top next time.
My two cents is to do it yourself. The reward of personal pride in the results is worth it IMHO. Don't underestimate the "I did it myself" factor!