Pro-Touring, Restomod, Custom
On the Corvette forum, pro-touring equivalent C1-C3s are generally called resto-mods. They usually have a custom frame and chassis, modern drive train, and a modernized cabin. A custom will generally not have a custom frame but a refurbished/reinforced original frame with modern suspension components added to the old frame. All will generally have new engines, transmissions etc.
In reality there is no official separation between the titles and many cars may fit all or most of the categories. You have to study the mods closely because many cars with a new motor are then labeled resto-mods by a car salesman and yet have few mods other than the motor.
There is a nice example of a well modded C3 in the C3 for sale forum with a 600hp LS7. He did everything but use a custom frame. On the C1/2 forum you can follow several builds and occasionally find one for sale.
What Ironcross said. They are all customs. You can go over to www.pro-touring.com and read the definition of pro-touring as uxojerry said. It's a sticky on the forum page. Restomod has been around for awhile. You can thank B/J Auctions for making that term popular. I'm old so I remember back in the day when Car and Driver built the "Blue Maxi" '69 Camaro with a LT1 and A/C with F60s and 15x8.5s American 200S wheels all the way around and called it a GT car. It was built by Penske Racing to be an American answer to the European GT cars of the day. Mark Stielow gets credit for building the 1st pro-touring car a white '69 Camaro that competed in the One Lap Of America. I think the Blue Maxi though could be considered the fore runner of what has evolved into the pro-touring style. The major difference is the make over was done to a "new" car at the time. I think the "pro-touring" title comes from the build style as being the counter culture of "pro-street". A street driven purpose built straight line single dimension car vs a car built to do everything well with performance, handling, and emenities of a new car. In the end I think you build what you like the way you want. Some of us are influenced by drag racing, some road racing, and others modern GTs. Let the critics call it what they will. In the end if it's not stock it's custom but you get to call it yours. 



On the Corvette forum, pro-touring equivalent C1-C3s are generally called resto-mods. They usually have a custom frame and chassis, modern drive train, and a modernized cabin. A custom will generally not have a custom frame but a refurbished/reinforced original frame with modern suspension components added to the old frame. All will generally have new engines, transmissions etc.
In reality there is no official separation between the titles and many cars may fit all or most of the categories. You have to study the mods closely because many cars with a new motor are then labeled resto-mods by a car salesman and yet have few mods other than the motor.
There is a nice example of a well modded C3 in the C3 for sale forum with a 600hp LS7. He did everything but use a custom frame. On the C1/2 forum you can follow several builds and occasionally find one for sale.
A restomod Corvette generally is designed for driving and road use. Its vintage look is retained as much as possible, but modern driving upgrades such as modern suspension, engine, transmission parts are the hallmark.
A custom basically has no such plan. Its mostly for show, and has no real "spirit" its trying to retain, especially in body appearance. Its a personal expression of style and execution and very individual.
Both generally have the higher budget which is what separates it from an ordinary modified car.
Both have lots of overlap and both cost plenty of dollars when done right. And of course, there are no rules or absolutes for any of this.
PS: of course all could be called custom or modified, but I think the terms are being used to describe different types of modified cars.
Resto-mod-Can be Pro-Touring, Pro-Street, or neither. The car is restored with no goal on keeping the car original. The looks of the car are typically original in looks but the inner workings are not.
Custom- Whatever you want the car to be, paint, interior...car does not look original
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