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With the headlight on 37 being original would it be a good idea to put a 6v relay into the headlight wiring and how would you wire it

I am a believer in relays to take load out of the old switches and provide a heavier gauge supply wire for brighter filaments but others do not feel they are necessary. Where and how you would add relays to a 37 depends on if the conversion to sealed beams was done or if it still has the 4 position headlight switch with the country, city, passing and all the other switch and filament combinations still working. If your car still has those I would forego the relay or maybe just have it when both bulbs are on high beam.

If you do have a sealed beam conversion with only a high and low beam controlled by the footswitch, here is a schematic of a relay setup that works well. It is designed for cars that have the terminal strip on the left fender where the individual headlight wires join with the main loom. The relay contacts get power from a heavier gauge wire to feed filaments and coils are controlled by the original headlight and dimmer switch so only the relay power goes thru the original switches. If your car does not have the fender terminal strip then how you would wire would need to change to accommodate the wire connectors instead of a terminal strip.

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Would you put a maxi fuse in line from starter to ammeter and if so what size fuse

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You could if you wanted to but I don't recall anyone mentioning they had done so. The amperage needed depends on how many accessories you have and how many are on at the same time. With a radio, heater, defroster, headlights and ignition all going at the same time worst case would be under 50 amps so for a fudge margin maybe go with a 60 amp fuse.

IMO, far more benefit would be gained by adding a quality main disconnect switch in the battery cable so you can kill all power when the car is parked or is long time unattended. Ideally it would be in the ground cable but since that is entirely under the car for 37 a switch in the hot side is the next best option. There have been some who mounted them on a bracket on the firewall a few inches away from the starter. Others have mostly hidden theirs by placing it in the floor near the battery just under the front edge of the seat so the lever could still be reached easily.

If you use a disconnect switch get a good one rated for 6v and capable of carrying several hundred amps continuous to allow for the heavy draw of the starter. Cole Hersee makes excellent switches available in many stores and some sold by Jegs (which may be private label Cole Hersee) have good ratings. Here is a Jegs offering with an appropriate capacity rating. Summit and Jegs has other good ones made by various mfgs.

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Thanks Howard I will be putting in a master cutoff switch Thanks for the reply