rehone, new rings, all bearings, & seals
cam regrinds, just some fast roaders and a 3 bar fuel reg should see some gains
lighten the flywheel and get the lot balanced with shotpeened rods and arp bolts and you should have a bulletproof xe
cost will add up, but i get all gm stuff cheap, so say a couple of hundred quid for the rebuild and another couple of hundred for the upgrades
I once enquired about rebuilding an XE bottom end at a local engineering place,SES Enginering in bolton.
A re-bore with 4 new pistons and rings to suit,new main bearings,new big end bearings,new small end bearings,crank reground,buitl up and balanced for just shy of £500 which i though was pretty good seeing as they built it up aswell.
As your engine is smoking though it would be a good idea to rebuild the top end,especially replace the valve stem oil seals.
Clean everything up,re-grind all the valves in,either strip and clean or replace the hydraulic lifters.
Blydenstein engineering do the best headwork on vauxhalls,he is an absolute god,he'll also fit uprated cams while he has the head.
Might also be a good idea to get the anti porous mod done while its all in bits.
The key to a good engine when its been rebuilt is running it in properly!