How necessary 5x7 Color Head for Cibachrome printing from 4x5 Tansparency?

I need any experienced large format Cibachrome printers to help me with a choice.

I already have an Omega 4x5 Pro Lab B&W enlarger. I wish to get a color head for both B&W and Cibachrome printing from 4x5 transparencies. I could replace the B&W head on the Pro Lab with a Omega 4x5 Dichroic head. The biggest problem with the Omega Prolab (Unlike the D5, for example)is you have to shim the column at the baseboard in order to get the lens board to align paralell with the base board. The alignment controls on the prolab where added as an afterthought in the design, it seems.

Or I could leave the Prolab to B&W printing, and start with a new color enlarger which is easier to align properly.

Finally, another one of the factors in my choice is whether or not using a 5x7 dichroic head instead of a 4x5 one will have any noticeable improvement in evenless of illumination on the edges of the tranny to produce better quality Cibachrome prints than from a typical 4x5 size enlarger.

I've printed a very few Cibachromes to 8x10 size using removeable filters. The light fall off effect was inconclusive because the subjects in the sample prints were dark at the margins to begin with. I've read that light fall off was a factor in serious largeformat Cibachrome printing from 4x5 enlargers.

Thanks in advance for your input.

P.S. I also wish to print from color negatives.

I print 16x20 Ilfochromes from 4x5 trannies using Saunders 4500II dichroic enlarger. No light fall off in my experience. Get a good lens (Schneider or Nikon).