After a full inspection of the boards, i can say the following: JLCPCB have done a fantastic job, for the money. The silkscreen on both layers is sharp and better quality than PCBWay for sure. Everything fits - although the holes in the prototype may be a *little* tight for reusing some parts - thankfully most of the parts that need to be reused are to be socketed anyway, so not really too much of an issue - although the SIMM socket pads were 1.4mm x 0.72mm, so i've made them and all the other DIP socketed parts a touch larger, at 1.6 x 0.8mm to make for a better fit, especially if reusing parts. The pads for the rear I/O, such as the ADB/Serial and the metal support bracket are a tight fit and i missed out excluding the pads on the underside from the solder mask, so i've already re-done the sockets from scratch to give a 0.25mm larger surface area (track width from 0.25mm to 0.50mm as they're 'drawn' and then infilled with an exclusion zone), as modern PCB's are very, very accurate and incredibly fine, compared to ones from the 1980's.
Now the biggest fear i had was hole alignment. Fear not - for all the holes line up bang on! Literally, 1:1 is what i was aiming for, and i assumed i may be out by about 2-3mm in some places, but from the measurements, 1mm difference at best - In addition, all the rear IO ports are PERFECTLY ALIGNED.
I'm going to start building these up on Monday/Tuesday next week as i have family commitments and a well earned barbecue to host. I need to get some parts from a US supplier of SIMM & PGA sockets - would someone mind being a middleman for that?