You have 2 easy options for manifolds to fit your H89 ovalport head

1 H95 inlet and matching H88 / H99 exhaust (this would leave plenty of room in the engine bay for a single exhaust pipe)
these manifolds do not bolt together so you could go for just the H95 inlet and custom extractors

2 G51 inlet and matching exhaust- ex A15 Vanette. These only work as a pair. they "drop down" which means the carby sits a lot lower - handy if you have bonnet clearance issues.