Yes..... probably worn suspension.  Can you post a side profile picture to show the stance.
The plastic cups that the knuckle joints sit in wear through and will drop the ride height, as well as flattened springs.
When you go to replace the rubber springs ( if they are original to the car....which isn't unheard of ) they will have fine imperial threads as opposed to later metric threads....just an fyi

I'd find a hill....drive up that hill in 4th with some throttle....a slipping clutch will slip for sure

You may have worn out suspension cones, probably should be replaced, also a good time to go through the rest of the suspension. Do you have hilo’s ? The height should be the same front to rear, would suspect the front has dropped some.

not a 12" wheel guy but search tires on this site in 12" for the mini. I'm thinking your tire profile might be incorrect. too tall.  The 12" tire is marginally larger than the original 10" diameter tire.

Wonderful advice. Thanks to everybody that responded so promptly. I tested the clutch with the 4th gear engagement while on the brake and hitting the gas. It stalled in under a second. So i am a bit relieved. I need to change the oil and I imagine this will help.

I have another question for you all. The wheel wells come into contact with the tire when I hit bumps or dips. It is obviously a clearance issue. I have 12 inch tires on the car. the side wall reads 165/60 R12 71H. The front wheels have exactly 2 inches of clearance from the wheel well and the backs tires have 2.5'' clearance. Could this be a misfit as far as tire and rims size? The tires are wide. Suspension issue?

Thanks again.

Do not overfill your oil.  Dip stick has two marks. Top one is full, second mark is one quart low.

Could also be caused by a leaking primary gear seal.

Sounds like a slipping clutch for sure

this is how you test for a slipping clutch.   Start the engine, put it in 4th gear and give it gas while releasing the clutch and holding the brake pedal with your gas foot.  At the friction point on the clutch the engine should stall immediately.  If not, time for a clutch.  Change the release bearing while your in there. 

Good day all,

I am a new and proud owner of a 1975, 1275 GT. I have had the pleasure of troubleshooting a couple of problems thus far. Nothing major, new alternator, rebuilt rear drums and will be re-charging brake lines shortly. My question is in concerning acceleration in regard to throttle load. As I work my way through 1st and second gear there is no problem. At the topish end of third gear the motor begins to rev high with what seems to be minimal acceleration. This is magnified in fourth gear. As i engage in 4th and press on the gas, the motor spools up in high revs but the car does not accelerate. It has more power to offer but only when i back of the throttle. It feels like something may be slipping? I am worried it is transmission, or potentially the clutch? The gear is fully engaged just no direct response between throttle and wheels.

Summation = Engine working hard, little to no power to the wheels in top end of 3rd and all of 4th when throttling heavy.