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Jul 21st, 2025 | | | | Joined: Jun 2018 Posts: 32 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2018 Posts: 32 | I have a 1947 GMC 300 with a 248 in it. I currently have a GKT-134 crash box in it.
Do I have any other options?
Im basically trying to decide whether I should keep the 248. If I can bolt up a more practical transmission that would allow me an extra gear or higher gears then I think I justify keeping the stock motor. Im just looking for a little extra speed. (55-60)
Thanks!
Logan
2010 Silverado 2500HD
1947 GMC 300
| | | | | Joined: Nov 2005 Posts: 6,061 'Bolter | | 'Bolter Joined: Nov 2005 Posts: 6,061 | Hy47GinPole, I know some of the larger GMCs used Clark transmissions 205V and 207V if I am remembering correctly, I'm not sure if a bellhousing change was necessary to install them or not. The Clark transmissions are "crash boxes" and they will not help increase your speed unless you find a rare overdrive version, 205VO or 207VO, hope that helps. | | | | | Joined: May 2005 Posts: 8,877 . | | . Joined: May 2005 Posts: 8,877 | A more modern overdrive option is the New Venture 4500, which can be adapted to fit, this may no longer be an economical option, they're getting old now.
| | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | The 248 will also drive a S-10 trans with an OD high gear, need make sure brakes and steering are up to higher speed, though.
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: May 2005 Posts: 8,877 . | | . Joined: May 2005 Posts: 8,877 | A 300 size GMC is more truck than a T5 from an S10 should tolerate.
I’d look for a transmission used in a 1 ton or perhaps larger. | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | GM 14 bolt axles give a variety of higher ratios, easily fit( look at Grigg's 14 bolt notes) are inexpensive, easily found; however, make sure your 248 can pull higher gears, might be a good excuse to fit a 302. Higher cruise speed might require upgrade to brakes, steering system.
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Mar 2010 Posts: 10,059 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,059 | Is taller tire height an option for a GMC 300? That would help.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Jun 2018 Posts: 32 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2018 Posts: 32 | What exactly does it take to do a NV4500 swap? Does the adapter have to be custom machined?
Also taller tires arent really an option. Its got 8.25-20's on it now which measure out around 37"
Logan
2010 Silverado 2500HD
1947 GMC 300
| | | | | Joined: May 2005 Posts: 8,877 . | | . Joined: May 2005 Posts: 8,877 | Start a new thread about an NV4500 swap, with a good title those that have done the swap will notice and chime in. Also search for past threads. Also look at “Advance adapters” they may have an adapter plate.
Several versions of the NV4500 exist, and they will adapt slightly differently. | | |
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