It sounds like this house has not had roof venting in 282 years. If the roof boards are not rotted, or if there is no evidence of excessive moisture under the roof boards, you may not need to vent. If it has worked well for 300 years, then why mess with it? But if evidence tells you you need to vent, then a continuous ridge ventwill help. For the eves you can use small round vents- just drill a hole and insert them.