i have a crx si so i have the d16a6. i'm using the stock a6 bottom with a y8 head, intake manifold, and throttle body. i'm also using a p28 ecu.
you don't use whatever arp head studs match the block, that's where some people go wrong. with the a6/y8 combo, you have to get a special kit from lightning motorsports which gives you 1 extra stud to use for the top left bolt hole that's a little longer, because the bolt hole there is a little deeper.
anyways, then you have to use a d15b7 timing belt because anything else will be too loose. you would use your same water pump and everything else for whatever block you're using, in this case for the a6. you must get new spark plug wires as the vtec wires are shorter or longer than any others, i dont remember exactly but you do need different wires. of course you're gonna want new spark plugs. and a head gasket of course.
i used the y8 throttle body which has the map sensor on the tb itself rather than using the one on the firewall. all you do is extend the wiring to use the one on the tb. or you can just keep using the one on the firewall, it doesn't matter if you leave one unplugged, just remember if you use the one on the firewall, you have to have a vacuum line running to it.
you're going to have to change the way that all of the vacuum lines are ran. there isn't a lot of them, but there just isn't the same combination of places to tap in on the intake manifold. it's easy, but you just have to play around with them. make sure you get several feet of vacuum line. another vacuum line you have to run is for the vacuum canister. whether you took it out or not, you have to run it to a port on the intake manifold. the other taps on the intake manifold are used for the fuel pressure regulator, brake booster, and pcv valve.
you're gonna need an intake and exhaust manifold gasket as well. the biggest thing is using the right adjustable cam gear. there are 2 different types. you have to use an adjustable cam gear with this setup. there's the z6 one and there's the y8 one. the difference is that the different gears are offset differently. if you have a z6 gear, i think you leave it at either 0, or advance it 9 degrees. with the y8 gear, you retard it 4.5 degrees.
if you're using the y8 manifold, be sure it's from a manual car, because then the iacv on the back will use the exact same 2 wire plug as your car. you're going to have to extend the wiring for that just a couple of inches. if you use the y8 manifold, you have to use the y8 fuel rail. you can still use the obd0 injectors and keep the resistor box. or you take out the resistor box and use obd1 injectors but you have to change the injector clips to use them.
also, the iat sensor and plug that was usually on the intake manifold of the a6 and z6 intake manifolds isn't on the y8 manifold, on the y8 motor it's actually in the intake tubing. so i just got the sensor from a junked car and ran it the same way. you just drill a hole in your intake tubing, install the right sized grommet, then push the sensor in. you have to extend the wiring from the driver's side of the engine bay to the intake tubing, which is less than 2 feet.
to swap to obd1, you have to get the conversion harnes. basically you're just going to run 7 wires from the ecu area to the engine bay. 4 wires for the 4 wire o2 sensor that you'll need, and then 3 wires to the vtec solenoid.
you'll have to get that ecu chipped and run the right basemap because the fuel and timing maps will be all wrong. i'm still not running right because i'm still in the process of trying to figure out exactly what map to run. my car runs pretty strong, but not as strong as it could/should.
going with the a6/z6 setup is 100 times easier and if i had to do it all again i would go that way. it's way easier and actually is better for making any kind of power. without getting into tons of detail about it, the head is better flowing and will make more power than the y8 head. not only that, but instead of having to mismatch everything to install the y8 head, everything is matching and easy to put together. and you can use a stock p28 which is what matches the head, rather than having to get an ecu chipped and finding the right basemap for your setup. either that or getting a chipped ecu and paying a bunch of money to get your entire car tuned. just think about it before you dump a bunch of money into your setup.