TBH, this is a tricky one to answer as it is not really clear on how you conduct your tests i.e.test object lighting conditions, that lead you to believe that the K1000 light meter 'seems to read accurately'
I would suggest that as you are going to be using a Pentax 67 for landscape work that you actually conduct a series of tests using your Soligor.
Some thoughts and information that I hope may be of some help:
1. There is a huge difference in both meters and this must be taken into account when testing.
The Pentax K1000 meter uses Cadmium Sulphide CDS which measures the average brightness of the ground glass using I believe the whole area to take an average reading (and integrates to grey?).
The Soligor meter uses a Silicon Photo Diode (SPD) which should be more accurate or at least able to measure in very low light conditions in comparison to CDS. In addition it reads a very small area that being as you know 1 degree.
Due to the difference at least in angle of measure you should take trouble to check the metering by from both is exactly the same. One way take a card white, grey or black (does not matter) and set the K1000 up on a tripod with the card absolutely filling the viewfinder with ISO set for 100 and note meter reading then with your spot meter to eye and on the same axis as the camera lens (as near as possible) make your reading. Ideally do this on an overcast day where light conditions not likely to vary over a minute or so
By how much do the readings differ?
2. Your comparing two different types of meters as well, relating to in camera or hand held and there are international standards for each. Additionally manufacturers are given some +/- tolerances by the standards so really you can expect some change between meters and systems. Exposure meters have an exposure equation including a constant 'K' (reflected) and 'C' (incident). The K being the famous or infamous and by some mysterious 'K factor'
The net result would seem to be exposure meters can use a percentage value for a mid tone grey equating to and varying between 10.5 - 14% (with possible a little more leeway). Leading to a possible difference of around 2/3rds of 1 EV.
Any more than this then you may perhaps want to get one or both checked out and perhaps the Spot meter may be the one to go for as it may prove quite useful for landscape work.
Richard Ritter, I understand worked with Fred Picker on the Pentax and Soligor meters and may be able to help (assuming still in business) with a service.
RTRITTER Large Format Camera Repair Services
3. If you are feeling adventurous/brave or are technically inclined you may consider having a play with calibrating the meter yourself if it is truly out and you can prove so with practical tests against a known source.
Underneath the label Soligor or Zone VI are 3 potentiometers which can be used to calibrate the meter each does a different job to bring the meter into line. I do have some information from someone who tackled this but do not have any method to prove the technique works. I will send you the information if you think it of some use with the caveat that any work you undertake will be solely your responsibility.
This is what you should find under the label
Soligor Digital Spot Sensor Meter Disassembly & Notes
In case you need the manual
Soligor Ditital spot meter, soligor CDs Exposure Meter instruction manual, user manual, free PDF manual, camera manual
I have a the same Soligor meter, somewhere, modified only with my own Zone scale and not with the Zone VI modifications. Although I understand that modifications to the meter was made which included IR, and a filter pack, to match the sensitivity to Tri-X film and some colour materials of the day the articles I have seen indicate that these applied to Pentax Spot meter with no mention of the Soligor.
Perhaps too much information here
But IF you are needing the accuracy and intending to follow the Zone system principles then a sound methodical testing procedure needs to be undertaken with the film/s of and developers of your choice to maximise IQ