Lowering and wheel/tyres 1970 GMC

hello, I am about to purchase a 1970 GMC shortbed sidestep. if the inspection of the car is positive, it will be shipped to Holland. the car is completely to my liking, I want to change two things in advance. the car must be lowered and I want to provide the car with different wheels.

Lowering: I was thinking of a 4.5 "or 5" lowering on the front and 7 "on the rear. The car has powersteering and a front power disc brake conversion w / complete front suspension rebuild.Rear brakes rebuilt, new coil springs & shocks front & rear, I don't know if the shock absorbers are suitable for a drop like this or if I need shorter ones I am looking for a suitable complete reduction set I have read this forum for a day or two, but there are quite a lot of different solutions and opinions I want to do a C-notch right away. I understand some dropspindles are better than others.

wheels: I have my eye on the 15 "Black Rock Series 942 Type D Matte Black Wheels. I do not want scrapping tires. I have read a lot about the rim sizes in combination with tire sizes. I understand that on the rear axle 15 inch 10J 4 inch or 4.5 inch Backspacing, with 295-50-R15 tires could fit ? I can't quite figure it out at the front, I read somewhere here that a 4.75 backspacing would be ideal. a 8J is available with a backspacing of 4.5 inches, the 10J is available with a backspacing of 4.5 inches.
I do not want the tires and rims to collide with the inside screens when driving in at maximum or against the wheel suspension. Can I fit 10J all around with 295-50-R15 ore is 8J with 265-50-R15 in the front the way to go ?

I have to ship all parts, so exchanging becomes very complicated.

I found rims on the summit website, if they also have good lowering kit’s, I order everything there. Then it can be sent in one go.

tips and tricks are welcome. I using this car for fun cruising, not on a circuit and not for any kind of racing. In my view the C10/GMC of this age look stunning with the right stance. The roads overhere are reasonable good. I also like the black steel wheels with white letter tyres.

Thanks in advance, the lowering package and wheel/tyre package gives me the opportunity to do it right in once. But the knowledge overhere is pretty limited. I have no unlimited budget to purchase a kit of $5000 and a wheel/tyre package of $5000.

Ron