I recall fitting an alloy P4 head off a 75 onto a P3 block, so that shows the 75 blocks were the same. The P4s used an SU carb. Easiest way to check is to compare head gaskets for matching bolt holes, cylinders, and water passages. Not sure if the heads off the larger 90 blocks will fit, try the RSR as above for info on this.
Edit :- the P4 75 (2103cc) had a bore of 73mm, stroke 89mm; the 90 engine (2645?) had the same bore, longer stroke at 105mm. Both had 4 main brgs, so I would suggest the bore spacings are the same. The alloy heads with SU carb should give you improved power - check the head gaskets - and contact some of the Australian clubs, all on the web. If you can get an alloy head on, think about next going to a later diff ratio, from 4.7 to 4.3 to reduce revs when cruising. I doubt the heads off the late engines would fit, as they went to 7 main brgs, which may have increaseed block length and bore spacing, and thereby bolt spacing. Loss of power in early motors could be caused by wear on the rocker shafts and rockers in the cam chest, which was not really overcome until the followers /rockers were fitted with rollers - 7 main brg versions. Locate the rear cover of the camshaft (left rear of block, level with rocker cover) and see if there is evidence of a hole cut in the firewall that lines up - easiest way to pull rocker shafts was through such a hole with an easy-out welded to a tube from inside the cabin. Much easier than pulling radiator and timing cover etc. This will release all the rockers for inspection.....but can you replace them? There are 2 shafts , a short one at rear, longer one after the distributor drive.
Probably more than you wanted to know?