Converting the car to a 4 speed is not that hard - you just need all the right parts. I don't know about increasing the value of the car - it's only worth more to you and anyone else who wants a 4 speed car and is not worried about originality.
You will need a trans, bellhousing, flywheel, drive shaft, transmission crossmember, pedals, Z-bar, linkage, etc. All of which are available new or used (here or on eBay).
Now is definately the time to do it. I converted my convertible to a 4 speed because I like it better than an automatic. I did save the original TH-400 and related parts - just in case I sell it and someone wants to convert it back.
