912 to 911 conversion.....pros and cons
 

i have a real nice rust free 1968 912 tired engine ,,,,,and i great running 1970 911t that was a light rollover........have the complete 911 suspension and drivetrain ,,,,,is it worth doing or rebuild the 912 original engine?

Lots of pros and cons to this question but I'll jump in.

First thing I'd decide is what you want to end up with, in other words, what is it you mean by "is it worth doing or rebuild the 912 engine."

If you intend to keep and drive the car, I'd say do the swap, especially since you have the parts, and costs will be mostly in time and labor (low, if you do it.) BTW, I believe SWB 911's had weights added to the front bumper to improve balance between front and rear. Something to consider if you convert and want your car to handle like a true SWB 911.

If you have an interest in resale (will you get your money/time&effort out) then I'd say do the rebuild. A clean, rust free 912 is in a climbing market and yours sounds like a prime example. I think these cars are underpriced, but catching on rapidly, and you have a potentially very valuable car if you keep it stock.

No simple answer. Maybe make a list for yourself of what you like and dislike about your options. Be honest with yourself, study the list for a few days, and maybe a choice will become clearer

I'd sell the 911 parts and rebuild the 912 engine.

If I was considering a -6 conversion, it wouldn't be with a 125 hp 2.2T engine. You can get ~200 hp out of those -4's now. They won't last long, but it can be done. So building up 125 hp on the stock case should be pretty simple to do and still have a reliable engine. You keep the stock numbers matching case, same hp as a 2.2T, and probably cheaper. Plus the -4 is lighter than a -6, so better handling, braking and acceleration. What's not to like? :D

Do the swap, its a great idea. Use an original year engine (2.0 aluminum). Otherwise, any flat six is far superior even with smaller numbers than a four. We all know that the number stats of a porsche engine usually have little to do with how the engine performs in a Porsche chassis and same power out of more cylinders will run more reliably. Plus, the flat six sounds good, the 912 engine sounds like a Briggs and Stratton (I still love it).