Why go to a 60-size engine when a 75-size engine weights the same, may be even some 90's. Look into it.
My friend put a tower hobby 75 on his 40-size Duraplane. It started to scream, unbelievable verticals.

Do you want to end up with only a 60 later on a 60 plane?

I think the tower's 75 will be cheap and give you 2.2 hp, that would be better for a 60 plane later.
For now the balance issue of that and the 60 will be the same.

Have fun, and remember, when in doubt: just go a little bigger.

I think I might have a few people screaming at me on this soon.

Later