oldie but a goldie

if it has an alternator it will most likely be negative ground

but if it has a generator it can go either way (positive ground is oldschool but isnt unheard of to still run into)
take the generator out and take it to an auto electric guy (starter and alternator shop)
he can tell you if it is negative or positive ground and give you some ideas where to start

you could probably convert the system to negative ground with a 1 wire hookup delco 10SI or 12SI if you do indeed find its a positive ground machine
again ask your auto electric guy about that

if you got pics of the engine and charging system you can send em to my email (in my profile) if youre not sure what you have

good luck