Keep the car what it is - A Z/28 !
Low optioned Z/28's are still Z/28's.
The other post about a cloned Yenko or COPO is correct. You need 5-leaf rear springs, special BB cross member, BB front springs, BB heater box cover, complete
BB radiator assembly, BB Z-bar assembly, BB 2-1/2" exhaust system, the complete engine with correct bracketry, etc., BB wiring harness, on and on and on......
There are way too many cloned Supercars these days for my taste. Keep your eyes open for the correct DZ block. One will show up one day, hopefully in a price
range that suits you. You can always restore it with another small block and go for the correct engine accessory items found on the Z/28. Then, as time & money
permit, look for the DZ block, build the long block and then transplant it into your restored Z/28.
The 396 is probably worth the cost of a DZ block and some internals of you're lucky.