After more research and talking to gm5735 I think all I need to do is use 2 x LH adapters and standard HK-HG 307/327/350 engine mounts and the engine will sit central to the engine bay. Will also mean that I need to move the gearbox rear mount across 10mm or so (will get a small footprint aftermarket one made with the gearbox centred rather than offset slightly). Should also mean some better tunnel clearance for the tailshaft too which won't be a bad thing given I'm using a cut down 1-tonner shaft and a 9". Once the front end is in place I can mock it all up anyway with a bare block and the TH350 to make sure the block sits level. If the mounts are not strong enough hopefully two LH side Tuffmounts can be used to replace the original rubber mounts and adapters.