I want to put up a 40m dipole antenna and my question is what a
transmission line to use, I have two defferent type of twin lead to
choose from, one is 75 ohm and a TV 300 ohm line. I'm using Yeasu
747
GX and MFJ 949 E for equiment. Thanks

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It depends on how you intend to use your antenna and how long will be
the feedline to the shack. You will be concerned mainly with the
inevitable loss in the line.

I assume one of your equipments is a tuner.

If you intend to use only the 40 meter band and the length of line is
not very long then the best choice is the 75 ohm twin line. It makes a
fairly good impedance match with both the antenna and the
tuner/transmitter at that frequency. A choke balun can be fitted
between the line and the tuner.

But if your line is a long one, then the 300 ohm line will probably be
better. It depends on how flimsy it is. Again use a choke balun and a
tuner.

If you intend to work on several bands then the 300 ohm line is a
must. There will be fairly high standing waves on the line at whatever
frequency you are on but losses may not be excessive due to the fairly
high line impedance. With 300 ohms it could be higher.

Ideally, for multiband working and a long line you should have a
substantial 450 ohm ladderline or, best of all, a 600 ohm open wire
line.

But don't neglect the antenna. The best antenna is the highest
possible.
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Reg, G4FGQ