[Everett#2390]

I would drop timing to 12* just to see if starter spins over better.
Checking timing marks is not a bad idea. However, I'd use a Sharpie to mark balancer. I personally never had any luck with tape staying on the balancer, even after cleaning with lacquer thinner.
It could be the mini starter may be drawing too many amps or cables aren't large enough gauge, min 1 AWG. Feeling of the posts and cables for heat will reveal too small cable(s).

Just my thoughts.

 

[Everett#2390]

Yes. I do know of a couple BBC guys installed a push button switch to open the ignition circuit when poushed so starter could spin engine over easier for a couple seconds, then let go to complete the circuit and engine started.

 

[Everett#2390]

The one wire from ign switch to distributor behind the dash, before the wire goes through the firewall connector, cut it, installed a switch, they used a "push-for-off" pushbutton switch in the wire and installed same switch on the bottom of the dash, right above the parking brake is a hole for conv top switch.

To start engine, push in on pushbutton to disable ignition, crank ignition switch to start, let engine come up to cranking speed, let go of pushbutton, completing the circuit, ignition voltage to igniton, engine cranks up. No starter drag.