Exhaust Brake on a 12 valve Hot Rod Cummings, manual trans?

If you are going to be upgrading turbos down the road, then get an inline EB that is down steam from the turbo. Turbo mounted ones are particular to a given turbo. As Russ said the exhaust valve springs have to be changed or you float valves. A push/pull switch on the shifter is popular. I would recommend one that works on air pressure vs vacuum.