If you know so little about tower work, the first thing I'd recommend is to get some help from a local ham who can assist you and make sure things are done correctly and -- more importantly -- safely!
The R5 vertical works about the same at any height above ground, and placing it atop a 45' tower won't really make it work any better than strapping it to your chimney or placing it on a short roof tripod, etc. The disadvantage of placing an R5 atop a 45' tower is that it's tall and a bit top-heavy and really creates a great deal of tower loading, for no extra results. A tower-top installation of an R5 also makes it difficult to service or adjust, and those are two things (service and adjustments) that an R5 definitely requires!
I'd put the Diamond VHF-UHF vertical right up on top of the mast above the tower, as high as it can go, and feed it with high-quality, low-loss coax. If you're going to install the G5RV as an inverted vee, with just a single high attachment point on the tower, that point can be anywhere below the Diamond, and of course the feedpoint must be supported away from the tower a bit to assure it cannot short out against the tower. The 450 Ohm Ladder line (31-1/2' of it) that hangs down from the center of the G5RV can just "drop," and you might want to stand it off a few inches away from the tower using PVC, wooden dowels, garden stakes (plastic or wood), or something, so the ladder line doesn't bang against the tower in the wind -- although I must admit I never bother doing this, and it doesn't seem to affect the way the antenna, or the ladder line, works at all.
I'd put the R5 on the roof of the house, or a porch, chimney, etc. and not on the tower. I've never known anyone to tower-mount an R5 and then not regret it.