Installed msd ignition box and dizzy. Now wires under dash try to get hot

That is not a voltmeter, it is an ammeter, which is in series from battery to load, meaning, BOTH those wires are HOT at all times and POORLY fused. Only protected by the fuse link in the engine bay.

READ this article for an overview on the how and why of this problem. You do NOT have to bypass this the way they did. Crackedback makes a nice little bypass kit

Catalog

The simplified diagram on that page gives a clear picture of the basic power distro

amp-ga18.jpg

The usual culprits are damaged terminals in the bulkhead connector, damage in out and around the ammeter and the wire ends, and in rare cases damage or failure to the "welded splice" which is a few inches from the ammeter on the big black wire, taped up in the harness.