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If you need a cylinder head, later XJ heads fit. They have an extension at the rear that overhangs the older blocks. It's not a problem, but exposed water passages have to be plugged. Also, you will need an inlet manifold from a straight port head (XJ or 420). In fact, the series 3 XJ heads are probably the most desirable.
Small hint regarding the water rail on top of the intake manifold. Do not try and remove the brass fitting on the end. It will snap off the end of the rail. I found out the hard way. Got a good used rail from a Jag restorer who said he never tries to remove them and just cleans them up instead.
Looks like a great trip. Portland is one of my favorite cities. Especially love the Spanish Coffees!!!