I would say it depends on your power goals. For instance, I am currently planning on staying 302 based motor in my '70. I might do a 347 stroker at some point, but I'm ok with limiting my power to under 400 hp (very safe range for longevity in a stock 8.2" deck block). For me with a convertible I don't want to get too crazy with power. If you like to turn corners - handling will be better with the 302.
If you want to race or have higher power goals, go with 351C. 351W is ok too, but it was built more as a truck/passenger car motor whereas the 351C was built by Ford for performance. Also, the Cleveland came as an option in the '70 Mustang, but the Windsor didn't. If you are planning to really push performance on a low budget, the W is cheaper to build with aluminum heads and all the extras. However, you can easily make 400-500 hp on pump gas with a Cleveland using the factory iron heads - that's an inexpensive option for good performance.
Since you already have a 351W, you might want to build another - especially if you have a lot of aftermarket parts on the one you are using.
The C6 is a good transmission. They are tough and inexpensive! The C6 does suck some power, but for pleasure driving it might not really matter to you. If you run a lot of highway miles you might want OD. Then there are several options as well which all depend on power. If you keep it below about 400 hp it's a lot cheaper to build an AOD that will last.