I would say that the SPAX shocks are pretty good, although the Koni types are rumored to be slightly better, I don't personally think that there's much in it.
I have adjustable SPAX telescopics fitted all round on mine, and they give quite a range of adjustment.
I find that a very soft setting on the rear is advisable to prevent hopping (bouncing), with a much harder setting on the front to prevent excess travel.
I would also consider uprated torsion bars on the front, as the car really needs it if you have uprated the engine and are travelling faster over bumps.
You can buy the rear types that fit to the original suspension points, but if you need further performance then you will have to fit the turreted type:
Which consists of a square 'tube' that sits in a hole in the rear floor containing the shock absorber, and has a bracing section between the left and right turrets which is also welded to the centre diff 'bump' in the floor. The top of the tube is blanked off and contains a hole which is the mounting for the top of the shock absorber.
The only trouble with this method is that quite a lot of welding is involved.
If you are going to all that trouble, then you may as well fit radius arms to locate the axle and prevent 'tramping' (axle hop), they also act as a rear anti-roll bar, and dramatically improve the handling.
The parts can work out quite expensive (hundreds of pounds).
For a supplier, give Jonathon Heap of JLH Minors a ring or e-mail:
http://www.jlhmorrisminors.co.uk/index.html
Sorry,