Thanks to everyone who replied with advice on the lifters. I've got a retired mechanic coming round tomorrow so I'll be able to let him see the link to the manual which Pre '68 Dave sent me, and starting with 3/4 turns first. The truck came from Arizona with the original engine which seems to have been uprated there before coming to Britain. It can cruise along at 65, but the fuel consumption is only 11 mpg, hence why the mechanic thought he could get it to run better by adjusting the lifters. Tomorrow I'll find out.
