Just measure a tyreless rim with a tape measure.

From memory HJ-HZ Polycast 14x7 rims were 115mm backspace and fit easily even with big tyres like 235/60/14 front and 245/60/14 rear. 7" rims with 100mm (4") backspace fits OK under the rear of a HQ-WB ute/van but will scrape under the front on lowered vehicles with a tyre like a 235/60/14. I doubt 100mm backspace on 8" rims will work perfectly on the back unless you have standard ride height. If I were you I'd run 15x7 front with the same backspace as a Polycast rim with something like a 205/50/15 (120mm backspace may work OK, you'd have to check). Rears you might get away with 15x8 with 120mm backspace with a tyre like a 245/50/15 or similar size but they aren't that common anymore. Wider stuff fits but they can scrape in some circumstances especially on the front. Normally once they scrape on the front they catch the guard lip that attaches it to the inner guard and can tear the tyre.