A few things.
You need to research exactly what the engine is. Somewhere on the internet is info on head heights and so on, the head may be a "normal" spit/dolomite head, or it may be low compression (not good) I think normal on a 1500 is 9.25, so that will be OK with a mk3 cam profile, although a bit more CR wouldn't hurt (9.5) if going for something more aggressive, you may want 9.75... Of course, I have no idea what your petrol is like, that may affect things.
The normal 1500 cam was an 18-58, and the mk3 was 25-65. There is a difference in journal size, but newmans, canley classics etc can supply the cams. Wade cams are in Oz(i think), if that is any help.
Lots of stuff here, although I think the data on the 1296 cams other than mk3 may be incorrect?? http://auskellian.com/paul/links_files/ ... _specs.htm
Re carbs. John Thomason did some detailed comarison many years ago between a single 1 1/2" SU and twins on a std 1500 engine. He optimised both on a rolling road. And discovered there was very very little difference in power output, and a little in economy. John Kipping (he is in New Zealand now, a real guru) liked the single 1 3/4" su setup on a toledo inlet manifold. Just food for thought...
I wouldn't bother with an external oil feed to the rockers. If you do, you need to blank off the oil feed between block and head, and restrict the new one down to 1mm or suchlike to prevent over-oiling and associated issues. The normal feed is perfectly adequate if everything is working correctly and not worn out.
Exhaust manifold. The 1500 has a twin outlet cast manifold also used on the spitfire 1500. a bit better than the std toledo single outlet one, but with a tendancy to regularly blow the manifold-downpipe gasket. I wouldn't bother with the change, if you do fit new studs and nuts, or better still, drill/tap the manifold to 3/8 UNC (or 10mm!) and use cap head bolts to fit the downpipe. That clamped it up much better and no more issues. I had also filed both surfaces flat as they were far from flat when I checked them.
Oil cooler a good idea,but make sure it has a thermostat fitted to prevent the oil running too cold (very bad) and indeed that is a good place to start.
The 1500 engine will never be one to rev, but is nippy as it has a great torque curve. Be aware of this, and don't go and try to build one that develops power at higher revs, it won't last well.