For that sort of car it'd make no difference. But note that HX body is fairly different (mainly firewall wise) to HJ, early HZ are much like HX, later HZ are different to early HZ and WB are even more different to the point that for example HQ-HZ buckets won't fit a WB.

As a real world example, when GMH Pagewood body plant built the last 600 coupe bodies late in 1975 (as bodies in white), they used a few up in late HJ. But the last 580 bodies were still around into HX, so to build them as HX's they had to remove the HJ firewall and replace them with HX examples. Point is if you wanted to use say a donor HX body to build a HJ, some of the original bits won't fit eg clutch mechanism. But for an LS1 conversion who gives a toss!