Apart from a dimmer I like to use light stands. That way I can raise or lower the light to help get the right temp. They also allow you to remove the top without moving the lights, which helps the life of the bulb and your heat source is never lost so your tank will still be warm when you put the lid back on.

In a room that hot a 75 watt bulb should be to hot. The temp on the side on the tank and the surface temp are two different animals. I found this out when I started using a IR temp gun. Get one. It will make your tank temps much more accurate.

In my 40 gallon tank I have a 75 watt ceramic heater on the warm side and a 60 watt day and night bulb on the cold side because I keep my house at 68. With the 60 watt bulbs hanging about 3 inches above the lid they giveme surface temps in the low 80's. With the 75 watt ceramic heater hanging about 6 inches from the lid it gives me a hot spot aprox 14 inches away of 90. Also the 60 watt bulbs I'm using are the regular style bulbs which throw the heat in a wide area. If your bulb is a basking bulb which gives a more beam like heat zone I can guarantee your surface temp will be way to hot. Again in my cold home a 100 watt basking bulb can give me surface temps near 100 at a distance of aprox 14 inches. I bet a 60 watt on a stand and maybe a dimmer too would serve you well.

I would get a light stand with a 60 watt and 40 watt bulb and see which gives you the right temps.