Query on Mounting Bracket on transom

[Gilaroo]

Hi,

I have a small boat 18ft with an inboard diesel but want to mount a bracket for an auxiliary 4hp probably 25-30 kgs directly on the back of the transom. I bought the bracket and the bracket plate takes 4 large 2.5'' size 8 bolts. I have aligned the bracket which extends down 3 levels allowing the prop to sub merge fine. Having a reluctance to drill holes in transoms, but having drilled the first hole which revealed the thickness of the fibreglass of about 7-8 mm, I just want to make sure Im doing the right thing and that the weight of the outboard on such a bracket will not put due stress on the transom. Given the lengths of the bolts I intend to insert a 1'' piece of hardwood on the inside of the transom as the thread only goes from the tip of the bolt for about 2'' so I need to buffer the other .5'' with something for the nut to screw tight. I assume putting the wood block on inside would also distribute the weight/pressure. Therefore I just wanted to get a sense and reassurance that this is the way to go before I punch the other 3 holes in the transom? The position of the bracket is well above the waterline and I intend to use a sealant on the inside and outside after mounting. Will a transom with this thickness support this rig ok ? Many thanks.