This question comes up often enough to merit posting. "How long should the MGA clutch slave pushrod be"? Since I had to crawl under my car and R&R the part for measurement, I just made a drawing while I was at it.

Note:   Push-rod length between the hole center and the spherical rod end will be about 5/16-inch shorter for Twin Cam and all round disc brake models. Overall length and outside diameter at the hole end may vary and is irrelevent.

Various Part numbers: 7H7954 Push-rod 1500-1600 13H21 Push-rod 1600 new number - 7H7954 not available use 13H21 BHA4074 Push-rod Twin Cam 1B3701 Push-rod All around disc brake model - Com.(C)91240 NOTE: BHA4074 and 1B3701 may be the same physical part.
1500, 1600, 1600-MK-II models	Twin Cam and all round disc brake models

See also Cutch Slave Push Rod for Twin Cam - TC-225