Three on the tree with a 318?

Hello "A-list crowd!"
I have a simple question,
I have a 225 six cylender three on the tree manual and Im wondering can I swap the 225 for a 318 without switching trannies, will the three speed hold up?
This 318 will be a stock 2 barrell motor with dual exhaust.
Also how can I tell a 71/4 rear from a 8 1/4 rear?
thank you everyone . love this site!