By now, I'm sure you've concluded that using an external converter with an 8031 or 8751 is not the "optimal" way to address this problem. There are many 805x-core devices that have on-board ADC's, as well as other cores, any of which would do the job with less pain than the way you've done it here. ISTR that there are some pretty small parts, e.g. 28-pin PLCC's that would do the job nicely, and even some in 16-pin or 20-pin packages, though many of those are not 805x-core devices.

Most of those will be considerably faster, often offering 12-bit converters rather than 8, and probably cost less than the combination of the MCU and ADC that you've presently got.

Interfacing external memory-mapped devices to the 805x series is not as easy as some other devices. Most, however, the smaller PIC's for example, don't even offer the option of using an external memory, though they do have some with multiple ADC's built in. In some cases, you actually get more than one converter, so you can sample all channels independently, thereby increasing the rate at which you obtain conversions. Some have built-in sample and hold amp's. There are lots of options.

Computing access-time requirements and designing your external circuitry to meet them is often a headache that internal devices help you avoid. Since the MCU doesn't sample the data until very late in the read cycle, there's no problem with the dual ALE's. However, on writes, the data is set up, then goes away during the second ALE/address assertion, only to return afterward. That can sometimes be a nuissance though I doubt it has any impact on your current situation.

Designing to the requirements rather than using some "general-purpose" design is usually the most efficient way to get the job done. If you're building projects for a class, however, it's often easier if you use a single circuit and learn how to adapt it to a variety of needs. It minmizes the overall amount of work you have to do.

RE