Building a big bore 420 from the old 350 motor will cost you plenty, that is for certain. I just built one so now my "cheap" pre78 racer isn't so cheap. Putting in a needle bearing cam is a must do I reckon. Have a look at most early XL heads and you will be hard pressed to find one that isn't flogged out on the cam end faces. It cost me around $700 to do my head. That's just for the machining which involved welding the the relief slot, (a common cause of head gasket failure), surface grinding, tunnel boring for the roller cam, refacing the rockers and bigger valve seats for the inlet.
Cam, Valve Springs, alloy retainers, slotted cam chain wheel, HD cam chain and big bore head gasket all came from XL-Int. I can highly recommend them as I had no problems with the service at all. Emails were replied to promptly and all questions answered. I had stainless valves made in QLD which again was no drama. I already had a big bore kit which I got with a heap of parts a few years back but expect to pay at least $450-$500 plus. You need to speed up the clutch gears as well to have any hope of the STD gearbox holding together. Clutch gears cost $400.
I was lucky enough to find a 1977 XL350 with only 1700 ks on it and clean oil in it so the big end and gearbox are all good to go.
I've made a 1 3/4 tail pipe but still need to get the header section made. No doubt another $150 bucks at least.
All up the motor build alone is around 3K.
Now I just hope that the build of the bike is worth it in that it handles well and does everything it should.
I will find out at HBBB!
