Currently the side walls of your beam are transfering the 1 to 1.5 tons of mast foot pressure from the underside of the beam to the top sit where the mastfoot fitting is. As you beam is rectangular this means the top and bottom planes will be pressed inwards and may develop micro cracks around the fittings.

Whith a large hole in the bottom of the beam the pin can pass straight through and transmit the laods directly to the underside of the mast foot fitting. That is one the advantage of the regular setup that you don't have anymore.

You may well be fine or you may not be with the current setup. Time will tell as I haven't done the math on your setup. Basically, keep an eye on it and if it is fine then you proven this setup to be a viable alternative.

Wouter