I'd recommend a CST spindle lift of 4" and a rear block of 1" I'd also recommend you go with their new line of cast spindles.
This will lift the truck and keep it level. This is from the Performance Lift website:
"We do have customers using the factory 20" Dodge wheel and up to 305 series tires without a wheel spacer. Please check for clearance before driving the vehicle".
I would buy the kit first, install 1 side since it really isn't that difficult, test fit and then worry about spacers if needed.
Also, I believe the stock 20" wheel has a 33" tire already on it.