I am building a bike but am new to this and need some advice. I have a Trek SL aluminium frame with carbon steerer. I have a Tiagra 39/52 chainwheel and an eight speed cassette from my old bike and want top know if I need to use a Tiagra braze on or band on front mech( 4500 model ) bearing in mind the downtube is 34.9, and also if a Tiagra short cage rear mech ( also 4500 model) is compatible. I haven't purchased theese items yet as I await this advice. Thanks for your help with this. Mike.