The distance between the bellhousing and the tranny mount are all the same on the TH400. Since you're going to need a new driveshaft anyway, find a short tailshaft version. Just about every truck TH400 will have a bolt on yoke and a 2 piece driveshaft. You'll need a TH400 slip yoke for a car which isn't usually easy to find.
As for internal parts, it depends what you want to do with it. GM changed a bunch on internal parts over the years. The late model TH400 (up to 1990) isn't as good as the earlier ones and will be full of metric fasteners (oil pan bolts etc)
Buy this book and go through it before looking for a tranny. It can help you in knowing what to look for.
http://www.amazon.com/gp/product/087...lance&n=283155 
