As a licensed electrician for the last thirty years, I'm impressed that there is a lot of solid info. in the responses, some incorrect, some dangerous (which is typical) most just fine.
First, the genny. Absent any extraordinary requirements, it should not be placed 150' away from the house, it should in fact be as close to the main panel as possible. It's all about line loss. Wire is expensive, and burying it in conduit ups the cost several magnitudes. When you do voltage lost calculations, generally once you are at the 175 ft. length on a wire run, you start to see excessive loss and need to upsize wire. Given the larger wire size required, and the distance, you are needlessly adding thousands of dollars to the job.
The second issue on the genny is a code required interlocking or load transferring device. One poster here spoke of just flipping a few breakers to cut the incoming power from the utility. Sorry, but no. As stated, there needs to be a device that physically and electrically disconnects the utility from the emergency power while operating the emergency system. Automatic standby generators use automated transfer switches that switch the loads that were chosen during the installation (heat, lights, fridge, et...) from utilty to emergency power. Manual transfer switches require that you flip each breaker to the emergency setting after starting the genny. Interlocks require the incoming main to be off, before allowing the genny backfeed breaker to be turned on. If you are going with a manual system, definitely use an interlock. Too many folks end up with a ten or twelve circuit transfer switch panel, and find that they would of liked to use other circuits in the main panel, but can't. An interlock allows a choice of every circuit in the house, limited by how hard you are loading the genny.
Finally, regarding generator safety. You CAN NOT run a gas engine in a building unless it has an expensive, complex, listed, labeled, code approved and inspected, exhaust to the exterior AND a make up air system. In the last decade six people in my area died, in two separate incidents when they had portable generators running at the threshold of an open garage door as they slept. A detached garage is not a safe environment to operate a gas engine either. You may be just fine with the garage door open, or you may attempting to accomplish some task in there while it's running and slowly drift off to sleep and never wake up!
Now back to the garage issue. IMHO, your sped past the two best options here. First, there is nothing wrong with an overhead line to the garage. Do some research and get a professional opinion about why yours happens to be an issue. There are requirements for every aspect, but chances are you could run an overhead sup-panel feed at a fraction of the cost of doing an underground installation. Do some google work and ask a pro about your options there. Second choice is a 100A service to the garage. This involves a new overhead drop from the utility, a meter and mast, and a panel in the garage. This is the best option if it's physically possible ( there is a pole located at a good spot for a drop, and/or the utility won't try to empty your wallet to accommodate you.)
You would end up with lots of available power, a completely new installation, no lost of breaker space and capacity in the house, and no expensive, ugly trenching. The only down side, as other's mentioned is billing. A demand meter, peak meter, time of day meter, or higher (non-residential) rates for a separate meter can be a deal breaker in some areas.
As for the specifics of any other issue here, not enough info. To go further online I would want to know more about the genny situation (why, what are your needs, do you want to run the whole house or address a very short emergency, etc...) More about the current condition of ther home's wiring, panel, etc... and the nameplate info. off the kiln. (volts, amps, circuit requirements, etc ...)
