HG, If you got 77mph without an overdrive then you were really going some! Even with that is pushing it I don't like to go that fast without overdrive and 3.54 diffs - both add about 30% to the original top speed of 60-65.
The manifold is an easy-ish swap. It bolts straight on and looks almost identical. The difference is the hole on the top for the carb is bigger and square instead of round to allow the double-barrels to mate. The exhaust one is also different where it mates to the inlet and so needs to swap which is a pain as you then need to adapt the exhaust...the 90 exhaust flange is a wider diameter. I just cut and shut a 90 exhaust downpipe onto a series one with a mig welder.
The other, possibly easier, carb option is ACR's SU conversion. That would no doubt run like a dream but I'm not sure of the efficiency. The only experience I had with SUs in the past was that they ran rich but that could have been poor set-up.
BB, sounds like your carb is in proper need of a service! Could be the big O-ring between the two halves is leaking?
Fitting the stuff from head and cam from ACR requires a fair bit of time and patience:
Radiator and preferably driver's wing off - I didn't take wing off and spent extra time ****ing around getting the distributor skew-gear back - shouldn't take long, except like me you'll probably end up having to fix the radiator panel, mounting brackets, captive bolts etc etc
Then undo all the plumbing to the top of the engine and remove distributor, manifolds etc
Whip rockers off, inspect and replace any very worn ones but only if they're very worn, replacement ones, even OEM can be a bit rubbish I've found, if they're just a bit flatted it might be better to dress them with a file if its not a lot of material?
Whip head off and send it to Roland at ACR for the swap. (don't forget the oil feed pipe at the back).
Have a look at pistons, not how much play, how scratched the bores are, consider doing rings then think sod it the compression was ok!
Take front off engine and remove timing chain and water pump, inspect all the timing chain bits and replace knackered stuff - a bit of wear probably doesn't matter but probably worth getting a new chain while you're in there.
The cam then needs to come out, take the inspection plates off, pull all the tappets and guides and consider replacing - I did mine but should probably have left them as one of the new ones failed after about 10k, remove skew gear from
Setting the new cam up is the most difficult part, there are instructions from ACR but you need a pair of dial test indicators. First find TDC and find it EXACTLY you want to be within a smallish fraction of a degree. I made up a pointer and circle that I nipped up onto the front pulley (I also realigned my timing marker while I was at it but it needs to be more accurate than that). It's easy enough to find, rest the DTI on the top of the piston and find its highest point. Then check either side of TDC by taking bearings either side (say 1/2mm down and 1mm down) and take the average of those - this is to try and counter any play in the little end or big end that might be affect the exact TDC you want to know where the crank is not necessarily the piston which might have a dead spot.
Then you set up the cam, this is a case of getting the #1 and #2 tappets to "rock" at exactly TDC, with both DTIs one on each you should get identical lift on them both. To set this up you will need to mess around with the chain position and the big cog on the end of the cam. The big sprocket goes onto the cam with a keyway which can sit in one of 6 positions each gives a slight offset.
That all takes an hour or two of farting around but it is worth getting the cam timed in properly. I'm half tempted to have an play with mine seeing what happens if you advance or retard it a degree, one should give more at top speed the other a bit more at low down grunt. However, where Roland advises certainly seems to work well.
While you're doing all this you also have to get the skew gear back in - this can be a pain as you need to drop it down a hole and line up notch for the screw that holds it in place, plus you have to get it up so that the keyway is pointing at the right point when the engine is at TDC plus you have to make sure its at #1 TDC and one turn round from where the valves are rocking or your distributor will be out by 180 degrees and you'll either have to swap all the wires round or have the dissy on backwards. This took me ages as I'd not removed the wing - in the end after nearly giving up I rigged up a web cam and computer so I could watch one hole while poking the other! With that set up I had it sorted in no time.
Then its a case of put everything back together, torque it all up nicely, do the tappets and see if it still runs!
When I did mine it took a good few evenings all in but a lot of that was horsing around with fixing dodgy body work, wiring and plumbing. The most involved bit if you're not technically minded or a bit cack handed is setting up the cam but is by no means difficult, just requires a bit of thought and patience.
I really enjoyed the whole process of doing mine and am really happy with the results. I'm still toying with the idea of doing it all again on the 109 but then again I may give it a 3.5 rover engine....
D