The 1500's used 4 different distributors, each with somewhat different characteristics. (You may want to check the Lucas part number on the side of the distributor to make sure you have a '77. The part number should be 41697.)
US-model Spits in 75-76 employed distributor vacuum **retard** in addition to the distributor's internal centrifugal advance, whereas the 77-79 employed vacuum **advance**. These all had the same Lucas 45DE4 type-designation, but they had different Lucas part numbers and their timing specs and advance curves (both centrifugal and vacuum) were different.
If you do have a '77 distributor, your 'dynamic' timing spec is 10 degrees BTDC for Federal models (NB: 2 degrees After TDC for California). These are crankshaft degrees, not distributor degrees, and are spec'd for the engine idle speed of 800 rpm and with the vacuum unit connected. At 800 rpm, the centrifugal weights are not supposed to provide any advance, and all the advance should be coming from the vacuum unit. The vacuum advance unit is supposed to be providing 16-24 degrees advance at 10" of mercury. This is quite a lot compared to other distributor advance or retard mechanisms used on the 1500s, and it infers your basic (i.e. 'static') setting is considerably retarded
Emissions removal should not affect this setting much, basically 10d BTDC is a pretty reasonable setting overall. As noted in other posts, you might want to experiment a bit if you feel performance is wanting, but I would not go more than 2 degrees up or down.
I would thoroughly check out how well the distributor advance mechanisms themselves are functioning if you're not satisfied with overall performance. If the centrifugal weights are sticking (or conversely if the springs have gotten weaker over the years) you will not get a very smooth advance curve from them. Similarly, if the vacuum advance unit is sticking or if the mechanical pickup plate inside the dizzy is sticking, you won't get very accurate changes. Sometimes the vacuum diaphragm itself gets damaged and you won't get reliable advance.