TOPIC: Drilling the Transom

I recently removed my 75hp Johnson Electramatic from my '66 Westfield. (It has a bad L/U). I clamped on my '59 Fat Fifty. I sealed the previous mount bolt holes with RTV silicone. If I were going to leave this motor on, I should probably drill new holes for mount bolts. Is it OK to drill MORE holes, in the transom? And is it Ok to test run with just the clamp mounts tightened in place? Thanks for any input...