Go bigger than 750 if you can-3 mates of mine run buggys for autocross,one is a 2l vauxhall red top,another is 1300 hayabusa and the third is a rage 1000cc blade engine. Believe it or not on some surfaces the 750 will bog down a bit. If you do go 750 or less(defo no less than 600)-you should run 10" wheels. Above 1000cc-12" or 13". Id also advise fabricating a few panels-thistles and sh1t are no fun flying in your face at 70mph. Agood thick plexiglass screen will help a lot too. And chain drives tend to break a lot-go for shafts if possible. Nice chassis work lad-best of luck