Your slow crank when hot is the most common electrical issue on classic and custom vehicles I see 20-25 of these problems every summer come thru my bays and even more people come thru with a starter in hand to be tested on my test bench. I have been rebuilding for 37 years and have specialized in classic and custom electrical for 21 years. My family has been in the automotive electrical industry since the 1940's. Most people blame heat soak of the starter and install all kinds of crazy heat deflecting shields without ever putting a meter to the system. The most infamous vehicle we have had through our shop had 9 starters installed by 3 different shops to cure the original problem. Each shop just kept changing starters, blaming their supplier until the owner got tired of it and moved on to a different shop. Most likely giving the impression that the last starter they installed solved the problem. Therefore they are probably still fixing this problem the same way. There are a 3 causes for your problem.
1. Internally in the starter. I always eliminate this the moment the customer tells me that the starter has already been replaced without fixing the problem. Typically old starters with worn bushings, brushes, spacers or insulators will cause this issue.
2. Battery cables. Most commonly with 6v is undersized cables. Something that gets overlooked with cables are the terminals. When teaching a class I tell people to take a magnet with them when buying cables. To test the clamp and lug to make sure they are not attracted to the magnet. Cheap cables use steel which is the worst conductor. You want lead, copper, aluminum, brass or gold. All of which are nonmagnetic.
3. Undercharging generator or alternator. 99 out of 100 people will never even consider this and it is by far the most common flaw that I see. 6v systems need 7.2v to 7.5v rate of charge for the type of driving most owners of classic vehicles do these days. Most trips are short 30 minutes or less around town and in stop and go traffic at slow speeds. Not 2 or 3 hours at highway speeds. Generator systems have different charging characteristics then alternators so don't be so quick to make adjustments or replace parts without being very thorough in your testing. Just as you check your starting system for voltage drop you should check your charging system for voltage drop. Generator charging voltage tends to fall off at idle. So when checking your system check at idle, at normal driving speeds, lights on and lights off. Check with transmission in and out of gear.