yeah you are in awkward territory. The 350c "can" be built, and the lockup function is nice ( had one in my SS ).
But th350 parts are going to be your cheapest way to go fast with minimal downtime from where you are now.
Using the same crossmember, shift linkage, and driveshaft *
*The bellhousing to mounting pad on the th350-350c are all the same. However most likely your 350c would be short tail so you would want to swap for a "short tail" th350. The th350-350c could be had in 3 different tail lengths so check before you swap.
200-4r will be more expensive to build. Bonus would be slightly better gearing, lockup, OD and re-use of your driveshaft.
Downside is it takes some fab work to get it all squared away with the TV Cable. Different Driveshaft, and it would need wiring in a toggle switch or aftermarket setup to control the lockup function.
th400 is stronger in every way to the th350 but heavier, need crossmember, and different yolk on the driveshaft. I recall something different about the kickdown as well
So depending on your needs, setup and budget I am sure we could all spend your $$ for you dependant upon your setup and budget.