Perhaps increasing blade tension and setting the guide blocks to a closer tolerance would help some, but the absence of a lower guide system might be tough to overcome.
Note: I would also try using hardwood dowel material -in direct contact with the blade, instead of brass, when attempting a close, critical cut. Bandsaws can be quirky and sometimes, subject to many variables, require intuition and developed operator skills [read: voodoo] for good performance.
Some setups were just not designed for accurate, demanding work.