[chumbucket]

I wouldn't be as concerned with weight but more so with the amount of power you're attempting to put on an old transom. Check the max horsepower rating as well. Not to sure on the '79, but I believe max was around 235. Which is a lot of power and gas consumption. How fast do you want to go, and how much abuse are you willing to subject your older boat to? I would be looking for something in the 150 - 175 range and that'll give you really good performance. A 200hp would be okay I guess but probably starting to push the envelope a bit. With a 150 Yammi, mine topped out at 38 mph and was still good on fuel consumption. Macojoe has a 175 Yammi and gets a bit more speed than that without burning much more fuel.
That's my .02