MG T-ABC

what poundage to screw up nuts after fitting exhourst manifold on engine

I haven't seen any figures for those specifically. I just tighten them good and snug, and check them again after the first couple drives. If going with an actual torque spec, I would use a generic one for a 10mm brass bolt.

Just for the record ...it's exhaust...

What is the thread size, about 5/16" fine thread?
Torque charts for 5/16 fine Grade 5 show 174 inch pounds if well lubed and 231 inch pounds dry.
If you believe they are not Grade 5 or equivalent, you could drop it to Grade A which is 73 lubed and 98 dry.

A little bigger, 10mm.

Ok, then torque charts for a 10mm x 1.25 fine thread Class 8.8 are 52 Newton meters if black untreated finish (higher friction), or 49 Newton meters if zinc plated (less friction).

What can sometimes be forgotten is that when these cars were made the quality of the steel used was inconsistent at best. Torque figures were seldom given.