The incandescent light sources are typically used with condensors. Without condensors and using a single 300 watt lamp then you are looking at major unevenness issues. A 300 watt lamp, in my opinion, is too low in wattage for an 8X10 enlarger I personally would look toward probably 1000 watts minimum (possibly higher) Even using an improper incandescent lamp with a condensor enlarger is asking for some problems for focusing the lamp.
When you mention a 138S conversion to 8X10 (using incandescent lighting)you would need to look to a much more extensive conversion. Because is it more then simply sticking a 300 watt lamp into a housing and expecting to achieve good results. A incandescent light source with condensors in a good quality system becomes a functional optical system collimating and focusing the light at the nodal point of the enlarging lens.
If it were me and I was going to want the best print quality, based upon my experience with Aristo cold lights, Saunders diffusion light sources, and a Durst condensor system, I would look toward purchasing a Durst 8X10 condensor enlarger. That raises the height and weight issues that you have already mentioned. I believe that it is possible to circumvent that issue as I have already mentioned. It would require modification of the column...depending on the model it would be possible in most cases.
In my opinion, considering a condensor enlarger, I would not buy anything other then Durst or Devere. They have optical systems that are well designed.
If you have the money and are really want the best equipment then look to Jensen Optical and purchase the new 8X10 condensor enlarger with the 85 inch height.
I personally think for my own printing that I would be very inclined to consider printing with a point light/condensor source as opposed to an incandescent lamp/condensor source. My personal experience places diffusion and cold lamps at the bottom of the pile...this is based upon my opinion after having printed extensively with all systems excluding the point light source.
The reason that the cold lamp is utilized for the 138S conversion is that it makes it easier for the conversion...not necessarily the best in my opinion.