i stole this info, from classicskylinesaustralia
Tap a hole the block for a timing belt tensioner
Gates make a timing belt to suit
RB25 NA pistons, for decent compression
All the usual block prep when building a fresh motor
Shave the head a tad for more compression
RB25DE Head (from R32-No VCT)
May need some oil restrictors in the head, they can flow too much oil and the head fills up with oil.
Remap an RB20 ECU or Nistune/PowerFC/other aftermarket ECU
Can use a later R33/34 head, you will either need to run an external oil line to run the VCT,
or weld the oil galleries in head and block it off - no VCT though - good thing to have particularly for NA application
RB26 head will work too - no VCT to worry about.
If you want VCT the R34/NEO head is best, gonna be pricey though, and I've never heard of anyone doing it. Should be pretty same same though.