Jerry,
Congratulations on getting hold of a Rustler. Should make a very good long distance cruiser.
You face the same conundrum as we did with Fantail - how to avoid messing up the sleeping cabin by putting a mast through it ? And I think the answer is the same, to rake the mast forward, and get the bottom of it into the heads compartment.
Putting it vertically through the foredeck seems to be too far forward for balance and for getting adequate bury, and also, the weight is rather far forward and anchor work will be more difficult.
Raking it (and offsetting it a little to one side so as not to impede access to the forecabin) would put it quite close to the existing mast position, and the heel would be on top of the ballast. Adding some laminations of ply and glass onto the deck, to stiffen it up, would maybe cover up the mess that will be made by taking off the bermudan mast step.
I have to admit that this sailplan seems to have worked better on Fantail, with 6 degrees of forward rake, than it does on Tystie, with 2 degrees of forward rake. Footprints is in the process of going from zero to around 4 degrees of forward rake, so it will be interesting to see how that turns out. I think it will improve her a lot.
I see she has 2.5 tons of ballast, not much less than Tystie, and she carries it lower, so will be stiffer. My mast is 220mm dia x 5mm wall, and I don't think you should go much below that - 200mm dia x 5mm wall at the least.