I dint repair my heatsheild - just bought a new one from Reno - bout £32 + vat or somthin....
I did the cable myself, but i ran it incorrectly aswell as damaging the heatsheild....it snapped 4 months later about 4 inches behind the fixing on the pedal. - i got a mechanic to do the job for me the second time - i swore too much last time....if you think youve run it through the bulk head wrongly, try a Haynes manual...i did but ran out of patience
If your heatshield is fine, dont bother with it, youve probably routed the cable somewhere it shouldnt be, and the heat from the manifoldit causing the cable to expand, and not working properly ie: going into gear....
As for the box, if the bearing is on its way out, you can just replace it...but its labour intensive so expect a FAT bill for that one!
If u wanna replace the box, i think the only differnce between the 16V and the williams are the gear ratios....so your power band would change, maybe not as fast off the mark but higher top end....id just fit a quality fast road clutch and recon the box if its gonna come out...unless your planning some sort of monsterous engine transplant??!?!?