You could sell your 5 speed and probably have change left over for buying an l18 and stumpy gear box ready to bolt in!

The 60 series 5 speed won't bolt up and it wouldn't be worth doing so as the 60 series won't last long. Lowest rated suitable would be 63 series- I've picked up SSS dog leg boxes for between $50 and $150 each. Good for l16. Dr 30 skyline stumpy more like $300 and good all the way to ca18 power levels.