Turn signal switch I would go with this:
http://limeworkspeedshop.com/newstor...&products_id=7
As far as Tranny and engine your questions aren't very clear. If using a 235 make sure it has hydrualic lifters usally came hydraulic with the powerglide the better model motor is late 58 to 62. read this:
http://www.patricksantiquecars.com/articles.html
If it were mine I would get rid of the heavy cast iron powerglide and go with aluminum case came on late 60's car's way better tranny. You did not say weather your truck has open driveshaft or closed (Torque tube) If it has closed driveshaft than you need to change the rear end to use aluminum case tranny. Good Luck