Found!
Thanks Lionel .... it has been a long time since I first got the vehicle, hasn't it? Still, making progress at a pace I enjoy, so no problem.
Mike, yes, the GP has what is in effect a 9N tractor engine but with some important differences. The 9N is a sleeved engine, whereas the GP is not. But the bore is 3-1/16 which you'll remember is the same as the 85bhp V8 sidevalve, so 9T pistons and rings, available up to 60thou o/s, fit nicely. The block is capable of being bored out to 60 thou o/s, and those 4 ring pistons and ring sets are available. When I pulled the engine down, that's what I found: 9T, 4 ring pistons had been fitted (and I have been recently told, were listed as a change to the GP parts list in 1943, via a respondent on the G503 forum). Other than that, main & big ends, cam shaft bearings, valves, gaskets etc are all 9N and readily available. It has those horrible Ford split valve guides and little horse-shoe keepers, and was originally equipped with non-adjustable tappets, but adjustable tappets are available.
The peripherals are also different: the tractor engine has a front mounted distributor/coil combination with no vacuum advance, whereas the GP has a right angle drive and conventional distributor (the coil is mounted on the firewall). The exhaust/inlet manifold is a downdraught, whereas the tractor is updraught, and the carbie is a Holley. So like the GP in general, some parts are easy to get, and some parts are very difficult and others are outrageously expensive: an original Holley carbie has sold for over $8,000 US!!
Mike
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