Well here goes... Aluminium frame router design

[JAZZCNC]

Just noticed this bit, It wasnt meant to reach the table because I was hoping to offset it with adjusting the spindle height and I left 150mm between base of gantry and M/C bed so I could hopefully fabricate a 4th axis on the unused part of the mc bed.

If I'm understanding you correctly then the Unused part is exactly that Unusable and putting a 4th axis there is pointless as you can't access it with spindle.? . . . . ( Edit: Ok Excuse that comment I'm not exactly firing on all cyclinders at the minute and realised you mean to put 4th axis head in that space and tail stock at other end running material down it's length.! . . . I was thinking material across the gantry.!!)

As for rest of machine then like I say it will cut aluminium no problem and if it's only occasionally and your not wanting perfection on the finish then it will be fine. If your wanting to cut deeper and get high quality finish the machine won't be stiff enough.
Personally I'd just go for it and if you find your cutting more aluminium than anything then build a machine just for this purpose because while a machine for cutting wood isn't 100% for Ali the same applies in reverse to some degree. Can't beat purpose built machines and Jacks of all trades always fall short in some department.

The bed and workholding on any machine can be a pain and often under estimated how important it is. This is esp true on machines that are intended to cut multi materials.
Problem comes from how you hold it down and the type of work and how you machine the material. If your cutting or drilling thru then having a nice aluminium bed isn't much use because it doesn't leave a nice warm fuzzy feeling when you chop into it.! So you need some spoil piece anyway and often this needs surfacing each time you use it if any decent accurecy is required.
Now IME the best bed is a combination of both.? So nice stable material for Machine bed that is surfaced parallel to the cutter with either all of it or part covered with sacrificial spoil board. If bed is large enough like mine is then having half n half works well and saves swaping between and surfacing for jobs that need the accuracy of the base.