The STC will be only $500, but it is paper only at this point. I WILL be exploring PMA parts in the future.
But it does not cover the 175... yet

A C-150 with a V6 engine? Do a mock-up weight and balance first. Take a standard 150's useful load, then subtract the weight of the O-200 and add the weight of the V6, gearbox and FWF accessories. Then add the radiator, water, hoses, etc.
You're going to be flying it using a Futaba transmitter.
When you contact the FAA, the first thing they will say is that you will not get any credit for the parts you did not build. So the Cessna wings, Cessna tail, and Cessna fuselage you are thinking about combining into a hot rod... will not be considered a homebuilt.
What a great way to spend ten or fifteen grand and never get off the ground.
Sorry to be an asshole here and burst your bubble, but there are a WHOLE LOT of other better choices. Wanna go fast with an experimental? A used flying Vari-EZE czan be bought for $12-15K and get you 160-190 MPH on under 6 gallons an hour. Wanna do super STOL in an EXP? Get a derelict Tri-Pacer fuselage and build a Bushmaster which will equal Super Cub performance. Lots and lots and lots of choices better than the "cobble an airplane out of Cessna parts" method.