I assume by turn signals you mean parking lights in the grill?
I have never messed with any of mine, I think they are glass, and I would check the usual vendors like Jim Carters for new ones.
The 8.25X20 tires should work great on the rear, assuming they don't interfere with the bed, I doubt they will.
As for the front, they will most likely fit and clear the fenders, but will rub the springs at hard turns. Many people don't mind this, how about you?
I will run 9R22.5 tires on my truck, they are the tubeless replacement for 8.25X20, I don't anticipate any major problems.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-