Peter,
If you have the timing cover and barrels off, and the tightness is still evident, it could be either the crank or cams that are tight. Try removing the front crankshaft nut and use a puller to lift the crankshaft gear a little - if the engine frees up, a shim is needed between the oil retaining plate and the shoulder on the crankshaft (some oil retaining plates have a step to provide the clearance). If the engine is still stiff, remove the camshaft idler gear, and check again. If the crank is still stiff, operate the clutch to pull the crank backwards, and then check with feeler gauges check for clearance between the front main bearing and the crank web (5 thou is ideal). If the crank is till stiff, the front main bearing is probably on the tight side (2.5 - 3 thou is ideal). Tight front main bearings have been known to turn in the cases - in which case, the locating pin deforms the flange of the bush, and tightens it even more. If the cams are tight, the cases will have to be split to effect a cure.
Regards,
Eddie.
P.S. While you have the barrels off, check for end float on the big ends. The front rod needs 10 thou or less end float, otherwise most of the oil will be thrown from that bearing and starve the rear big end. If your engine was built from spares 30 years ago, it probably has a reconditioned crank so it's length may be a little bit suspect, and need re-shimming.