Cheapest to modify is probably the 617. You can get it to 603 power with minor mods.

The 603 is also very modifiable, use a later head for more power.

THe 606 is the most expensive to buy and to modify, but being essentially a 603 design with a 4-valve head it starts out with better breathing and gives you more headroom.

If you end up in SW Michigan I can show you around a partially assembled 603 with minor mods.

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Gone to the dark side

- Jeff