Did this company make more than one style of head? The one I have appears to have been made for an engine with large valves. The head has never been modified in any way, and is in perfect condition.
Post some pictures
Giant Power Head
Licensed under patent 1645423
A quick “Keyword Search” brings up a bunch of threads on the subject.
This one has photos and a link to the patent . . .
I'm presently running a Giant Power Head on our 13 touring. It performs better than the low head and holds more coolant. Has lower compression compared to the Z head. It is cast iron vice aluminum which I like. The engine in our 13 runs cooler using the Giant Power Head compared to the stock low head.
The Model T club of Tulsa compared several types of accessory heads some years ago - here are photos of a Giant Power head among others: http://mtfctulsa.com/Tech/head_photos.htm
"The Haibe Hipower, Giant Power, and Simmons Super Power Heads all appear to be similar, except the Simmons head has the spark plugs over the intake valves. These photos are from Dean Yoder."
Comparing the combustion chamber between the Giant Power Head and W/R head....with the heads sitting next to each other....they look identical.
Comparing the combustion chamber between the Giant Power Head and Z head....the valve pockets are much deeper/larger in the Giant Power Head. The deeper/larger valve pockets reduce compression but would work well with larger valves. Both have the horse shoe style squish area. Changing only the head in our 13 touring from a stock, milled low head to the Giant Power Head did provide better pulling power going up hills. I did not notice much difference in compression between the two heads when hand cracking. Did not do a comparison on top speed as I will not drive our touring over 40 MPH.
Have also owned a Reeder head which provides very little valve pocket area restricting valve size/lift. The Reeder head has a flat combustion chamber.
I am in the process of building a replacement 13 engine for our touring and will be using a Z head. I feel the Z head is the best bang for the buck....just wished it was offered in cast iron.