I mean... you could easily try putting the gearing on and then installing the rear wheel with the chain tensioner and see what sort of slack it takes up? From there you can decide if you'll need to take links out. Unless you're trying to avoid purchasing the new ring and cog altogether?

Or, you could keep your big ring and use a 16T on the rear. I just calc'd it with "standard specs"

Wheel size: 27in nominal
Crank length: 170mm
Front: 46T
Rear: 16T

Gives you: 77.6 Gear Inches.

Change that to:
Front: 42T
Rear: 15T

Gives you: 75.6 Gear Inches. Almost the same... ur tensioner would almost certainly pick up that slack.

Edit: Do you mean chain tensioner as in Single Speed derailler style? Or did you mean chain tensioner like... a surly tugnut for fixed? It wouldn't make a difference in anything but skid patches. Maybe not even that... I don't happen to have that chart right in front of me.