" Anything technically that I should be aware of if I choose 18 inch wheels and tires?"
Getting the offset right is the most important. I use a plumb bob and measure the things the wheel will rub. Rear- Leaf spring is usually the limiter inside and the quarter panel inner sheet metal is the outer limit. Remove the tire/wheel and using the plumb bob mark a line on the floor at those spots. Then drop a line off the wheel mounting surface (brake drum or rotor).
Now you have your section width and offset in the rear. Subtract 1/2" x 2 = 1" if you have a rear sway bar, 3/4" x 2 = 1.5 inch if you don't. Get custom offset wheels to center the tires in the available section width space.
Fronts - you have to make a card board or plywood wheel to measure with after picking the diameter tires you want. 18" is likely to be a 26" to 28" tire. Bolt the cardboard or plywood wheel to the front after backing off the torsion bar and setting the ride height. From the plywood, measure the distance to the inner sub frame and inner wheel well at both lock to lock positions. Again, use the wheel mounting surface to determine the center mark on the floor for front wheel offset and subtract 1/2" from inside and outside measurements for tire clearance.
Also, be aware that an 18" rim could touch the upper ball joint if you get unlucky. Use the mock wheel to check using the wheel makers dimensional data.
It's a lot of work, but allows you to stuff a lot of rubber in a stock wheel well. 18x8 and 255/45/18s with 5.5" backspace
