I would like to put 700c wheels on my 79 or 80 motobecane grand touring. What are my options if i want to use re-use my cassette?

your question is incomplete.

what wheels do you have currently?

if you have 27" wheels, and the same spoke count you can reuse your hubs, and lace them into 700c wheels.

you may or most likely will have trouble with the brake reach.

i would also guess you don't have a cassette, but a 5 or 6 cog freewheel (with built-in coasting mechanism) that threads onto a rear hub. if that's the case, simply buy a 700c rear wheel or hub with the same standard threading (english, most likely) to receive the freewheel. if it were me, i'd go to a local shop and ask to borrow a 700c wheel to see how my current brakes reached the new wheel. it's helpful to do this both front and rear as you may find the smaller diameter fits one of your brake caliper's pads better than the other.

The brakes are probably Weinmann 610 (short reach). Equivalent Weinman 750 brakes wouldn't be too hard to find if the 610s can't reach the 700C rims.

+1 Your bike does not have a cassette, unless someone upgraded the wheels earlier.

By 1979, Motobecane was moving to Swiss threaded bottom brackets. Absolutely no impact on a wheel change, but will affect choices if you decide to upgrade/change out crankset.

Is there something wrong with the 27" wheels you presumably have? If not, I don't really see a compelling reason to switch to 700c. If you need new wheels anyway, you might as well go to 700c for the greater tire selection. You'll also pick up some fender room, but on that bike I suppose you have plenty already.

Is there a particular reason you want to re-use the freewheel? If it's the original one I'd suggest replacing it anyway, and putting on a new chain at the same time. You can get a modern Sunrace or Shimano freewheel and your shifting will improve significantly.

thank you all for your very informative responses. Yes I am a newb. I am taking my bike to hawaii, and the aluminum wheels(mostly the rear) won't true very well, at least the bike shop thinks so. I wanted to be able to run a more aggressive cross type tire and have higher pressure as my bike gets heavy loaded down.
I have a pretty good deal on
https://www.sun-ringle.com/product-va...elerator-x2-0/

and

https://www.sun-ringle.com/product-va...elerator-r2-0/


Pretty good deal like half off. what is the difference between a fee wheel and a cassette? I see mine threads on and cassette seem to "pop" on? Im guessing these wheels won't easily go under the motobecane?

Thanks again for your help

Main problem with those, for your case, is not the freehub itself, but the fact that the axle width is probably 130 or 135mm, vs 120mm for your Motobecane rear dropout spacing. There are many nuances to exchanging rear hubs & axles, unless you do like-for-like.
An easier and possible cheaper replacement path for you would be a set of vintage 27" (less expensive) or 700C (more expensive) alloy wheels, either from the for sale sub-forum here, or ebay. You would be best off with an axle width of 120mm, or max. 126mm.
You may also be able to find narrower (120mm or 126mm rear) new wheelsets, that take threaded on derailleurs instead of cassettes.
There is also the option of spreading your rear triangle to accept a wider, modern axle/hub like the ones you linked, but that is not a decision to be taken lightly, and is a procedure best performed by a professional or experienced amateur. Then you will also have to determine how many rear cogs you want/will fit on your cassette, and what shifting system to use with them.

https://sheldonbrown.com/free-k7.html

Thank you for the informative reply. I am also doing a lot of research on the basics but you basically covered all i need to know.
Great forum members here thanks again.